Zirun: Making a game DRM-free and cross-platform isn't something you decide on a whim. It's something you commit to from the beginning. If you decide later to do so, it costs a lot of time and money to fix, update, and test.
THQ is a AAA game creator. The games cost millions of dollars to produce and update. They're also a struggling company. Making their games cross-platform (and possibly DRM-free, as I don't know exactly the details about whether or not Steam has consolidated code for developers to use or whatever) would cost millions of dollars that they literally can't afford. There's no way, unless the games were already cross-platform, for THQ to make them so in their current situation.
Humble also happens to be by far the best-known bundle creator. They didn't choose to make the Humble THQ Bundle on a whim. The guys at Humble are also caring people, realizing that THQ needs help to keep afloat, and that they are probably the best option THQ has.
So either Humble rigidly adheres to the rules they have and the Humble THQ Bundle doesn't happen, or they make an exception to help a struggling company that delivers games everybody loves. I don't see why they would or should indiscriminately apply their rules to everybody in every situation, since every situation is different.

Endzville: Said it best. The over-reaction here is outstanding and the attitude quite frankly disgusting in some cases.
Maybe I'm wrong and they'll have a similar bundle in the future but I would be very surprised if it wasn't with at least some good intentions like this - i.e. supporting a developer who desperately need the help.
Kristian: Zirun, other development studios have done just that after being required to in order to be in a HIB. Like Double Fine Productions with Psychonauts. The game didn't have a Linux version before it was in the HIB, I don't think it was DRM free either before that. But they made that happen for the HIB, now no developer will make that happen because of the HIB.
Endzville: You miss the point of what he said. Do you think Psychonauts was brought to Linux and lost its DRM overnight? Of course not - it was planned well in advance of the bundle's release. As Zirun said, these things aren't decided on a whim. They take time and money, things that THQ unfortunately do not have.
The decision may well have been done well in advance but that was because of the bundle, now we will cease getting any new games like that. Going forward all the game going forward that will be DRM and cross-platform in these bundles will have been so before being approached by the Bundle. From this point on we will cease getting cross-platform, DRM free debuts and that is bad, very very very bad :(