I remember saying the exact same thing here on HN like two weeks ago, which someone then promptly corrected me saying that Steam/Valve actually "explain" what their "Buy" means right before payment, and I think they were right, there is some greyed out text somewhere explaining you don't actually "Purchase a copy of the game" but you license it via Valve/Steam somehow, can't remember the details atm, later at the checkout process though.

>greyed out text

You mean the full color box thats for all practical purposes the same size and prominence as the cart you're checking out?

https://imgur.com/a/0fkh4l3