Whenever I hear the term “free trade”, I tend to think of the maniacal and chilling final scene in Paul Thomas Anderson’s There Will Be Blood. The oil industry epic ends as Daniel Plainview, played by Daniel Day Lewis, has gone from frontier pioneer to deranged, drunk robber-baron, devoid of conscience. He has sucked the land dry of oil and cheated preacher’s boy Eli Sunday (Paul Dano), the more honourable side of capitalist enterprise.

“Here, if you have a milkshake, and I have a milkshake, and I have a straw,” Plainview intones at the crescendo, “There it is, it's a straw, you see? Watch it. Now my straw reaches across the room and starts to drink your milkshake. I...drink...your...milkshake. I drink it up!”

This bit of unforgettable cinema more or less embodies everything I associate with the “freedom of the marketplace”. It starts with the lure of dream and adventure with free and open contractual exchange; it ends in the psychotic and murderous pursuit of power and control.

And I wasn’t alone. This was pretty much an orthodox view on free trade within the broad left.

But now, it seems, I’ve missed a memo. Because free trade has been recoded, not just as somewhat progressive but as actually, actively left-wing. This brings me to the March for Independence and its pro-EU agenda. But specifically to the co-convenor of the Scottish Greens, Lorna Slater.

To her right wing critics, Slater is an unhinged leftist who has somehow infiltrated the heart of government. To her supporters, by contrast, she embodies a radical spirit thwarted by the powers that be.

However, an examination of Slater’s words reveals a thornier dilemma. Last week, in the Edinburgh Evening News, she declared that “freedom of movement and trade in the EU were among the greatest political achievements for generations”.

For all the blandness of this phrase, it’s actually an extraordinary victory for the ideology we once called neoliberalism because Slater is both self-consciously leftist and seen as left-wing.

Yet the institution she praises as the “greatest political achievement”, the European Single Market, was designed mostly by Thatcherites to order to bend the emerging European Union in the direction of Anglo-American freebooting capitalism.

It's one thing to claim that the European Single Market is now the best option in a world of difficult choices. That’s a pragmatic debate about the limits on national sovereignty.

But Lorna Slater, as the quote makes clear, is going much, much further than that. It’s institutional cheerleading: free trade isn’t just a choice fatalistically foisted upon leftists. No, it’s now the “greatest achievement” that we must champion and defend.

It all makes me feel a little bit Grampa Simpson.

What about freedom of movement for people? Slater’s supporters might claim that she is defending free markets in pursuit of a socialist goal of open immigration. She told the assembled crowd on Saturday that independent in the EU, Scotland can be more “Kenmure Street than Downing Street”, referring to the community-led protest in Glasgow’s south side against the immigration removal of two Indian men.

In truth, this reading only redoubles the problem, turning Slater’s position on independence in Europe from cheerleading into mythmaking. People are perfectly entitled to agree or disagree on the merits of the European Union. But the ideology of “Europe” in the minds of Independent-in-Europe leaders bears almost no relationship to the true reality of EU politics.

European member states, for the most part, are governments of the centre-right, sometimes now of the hard-right; and even the small band of centre-left leaders can be quite resolutely anti-migrant.

More importantly, the EU itself has long defended “Fortress Europe” policies to keep non-white, non-Christian migrants at bay. When it comes to Kenmure Street, I’d wager that the likes of Frontex (and, let’s be frank, our own Police Scotland) would surely be in institutional solidarity with the Home Office, rather than the protesters, in an independent-EU Scotland If the EU defends workers at all, it is a protection racket for white labour against migrants from the global south. Hence the funny little secret of Brexit that no side likes to mention: non-white immigration has increased, proportionately, since we left the EU. Indeed, the worst of British policymaking, such as the Rwanda flights, are more or less rip-offs from the EU policy of offshoring migration management.

But I’m aware that I won’t change anyone’s mind on the European Union and that okay: hearts have long hardened on that question. But what really interests me about the Slater case isn’t to argue that she’s wrong, but to investigate the peculiar habit of ideological “re-coding”.

The EU was once coded as right-wing and opposed by socialists – Tony Benn, etc. Over time, it was rebranded as a sort of technical necessary evil, devoid of ideological substance.

Nowadays, the very same institution – which, over time, moved in a more economically Thatcherite and socially conservative direction – are furiously defended by people who, in all sincerity, believe themselves to be on the left.

This ideological re-coding is weird by itself. But what’s really disorientating for many of us is the absence of historical perspective, accompanied by a high-handed, hectoring tone. There’s not even an effort to rationalise this turn in thinking.

Back in the day, the left opposed free trade as the ideology of rapacious capitalism; nowadays, we celebrate it uncritically, as the foundation of all liberty and progress. There’s no mediation, there’s no narrative of that journey: it just happened, and so, here we are. There’s just right thinking and wrong thinking.

Truthfully, the rightness or wrongness of Lorna Slater’s political positions are really besides the point. It disguises the real puzzle: how did cheerleading for the world’s most powerful institutions become the very purpose of so-called radical activism?

Somewhere along the line, somebody has drunk our milkshake, and we’re barely even conscious of it.

Cat Boyd is a Scottish trade union activist and a co-founder of the Radical Independence Campaign and RISE – Scotland's Left Alliance