Whenever you make digital purchases through steam (or Amazon, or iTunes, or Origin, etc.) you don't actually have legal rights to the game itself. You are paying for a license to play the game. Technically, Valve/Apple/Amazon/EA etc. owns the game and has the authority to withdraw your licenses at their discretion. On top of that, I think most companies (not Steam as far as I can tell) have clauses in their EULA's to prevent class-action lawsuits that would sue them for withdrawing licenses that were paid for if they ever go under and can't support their communities. It's different than actually owning the discs and having physical copies of the games. Read the ownership section here
http://store.steamgames.com/subscriber_agreement/
DISCLAIMER: I'm only recently becoming aware of these things. Apparently, EA has received a lot of shit through Origin after the release of Mass Effect 3 and it got me interested in digital rights ownership. My post may have some fallacies. Be gentle, it's my first time.