None since its a Steamworks game and everyone accepted Steams ToS. If they don't want their files modified they have the option to never update the game.
This isn't strictly true. An agreement can be considered unconscionable, and there isn't a lot of legal precedent regarding shrink-wrap licenses or EULA for someone to make a claim one way or the other. If, for example, in the ToS for Steam, it wrote:
"by accepting these terms, you agree to give Valve and its partners full access to the files on your harddrive"
a court could still rule any such access illegal, the same way they would obviously rule a murder illegal if you were to accept a EULA that wrote:
"by accepting these terms, you agree that Valve can murder the **** out of you whenever it wants."
There are also certain rights that cannot be abridged, even in contract. The U.S. acknowledges most of these (though I believe they're saying "**** it all" to privacy). To summarize:
1) The contract could be determined unconscionable, but there has been no real legal precedent to decide on this vis-à-vis shrink-wrap licenses and EULAs
2) There are certain things that cannot be enforced by contract, period.
So it's not as though "but you agreed to it!" is a 100% foolproof fallback. Gearbox will still need to watch its step, especially with more sensitive issues like privacy.