Maybe, but this principle feels still reactionary in this case, depending on your presented solution. For now, we are more progressive towards our communist goals and better off with the self-checkout. It is a more centralized and efficient option in general. The problem isn't the fact that we are doing work for free that someone else should be paid for, it's that we should socialize the results of that free work in dropping prices and investing that labour elsewhere. Otherwise we're just re-privatizing the half of a process that's closer to socialization.

I don't think I'm necessarily complaining about your position here, you could just be using this phrase and agreeing, but I often see this phrasing followed by the reactionary "we can finally have normal lines and clerks like we used to have". Giving them that job back now (in full amounts, like before self-checkout lowered the amount) just results in even lower pay for those people and decreases every other benefit from it. There's never a going back in these cases which won't result in much worse things and further from a workable position strategically..