You typically don’t get “ownership rights” when you purchase a game on Steam. You’ll typically be purchasing a licence to play the game, which could be taken away at any point.
That is certainly what Valve thinks and writes in their TOS but if their store has a big button that says "BUY HALO" then courts may very well decide that you actually bought Halo.
And many countries have a strict legal definition of what buying means that cannot be overruled by some company's TOS.