They have no obligation at all; the letters are requests. However... if they don't reply, the committee will likely assume the worst about their privacy practices. It's probably in the developers' collective best interests to convince Congress that some amount of self-regulation is occurring. (The last question is even "(9) Please list all industry self-regulatory organizations to which you belong.")
In most cases, this is true. In paranoid cases, however, I suspect Ken Thompson would disagree with you.
If you click through to Hickson's actual summary, you can see why Microsoft is being largely omitted from the discussion:
"Microsoft has not commented on their intent to support <video> at all."
I don't think I'd want to risk my account to dungeon dive without spoilers. Then again, I was never that good at Nethack. Hm, does the explore mode switch work here?