I bought the game on Steam in 2014 because it was cheaper than anywhere else at that time - but if that means I'm forever tied to Steam for any further purchase regarding ESO, that's abusive. If you're forcing me to buy from one retailer instead of having a choice, that would definitely be illegal, at least in Europe. Are you saying that I 'should' buy it from Steam or that I 'must' buy it from Steam ?.

But I still want to know whether I can buy the expansion wherever I want or if I am forced to buy it through Steam, because I bought the original game from Steam in 2014. So does that mean if we buy it, we can't link it to our Steam accounts? That edition is not for sale in the Steam Store. We're Steam users who want to be able to buy the $100 physical collector's edition and link it to our Steam accounts. I don't think you understand the question here (at least the question for some of us). We have a small help article that will direct you to the right place:

If you purchased ESO through Steam, you should get the Morrowind upgrade on Steam as well.