No one's forcing you to actually get the DLC, you know
Besides, it's not like these are the first games in the Humble Bundles to have DLC.
Exactly, you don't need to get any DLCs for any of the games if you don't want to - they are all good on their own.
Anyway I don't see anything suspicious about it at all, I think it pretty much comes down to:
1. All games have been out on steam for a long time, they've all been ~$5 or so. Anyone that's going to buy them for a reasonable price probably already has. This way, they can get a few bucks from people who want to pay less then $5 per game, without making some kind of insane 95% off sale on steam or something.
2. Some people pay lots and lots of money for humble bundles because they
are crazy like to support the concept. Everyone who pays $1000 for it is just giving THQ free money (assuming they don't drop the THQ slider down).
3. All of the games in the bundle have a sequel that's out right now, or will be released relatively soon, except red faction armageddon. More people playing the original = more people who might buy the sequel.
4. THQ really needs the money, like REALLY needs it. They are not doing well, at all, they need anything they can get. The bundle has made ~$3 million so far, at the default split of 65% to THQ that's almost $2 million... Considering THQ's market cap (one way of judging the overall value of a company) is a pitiful 9.94 million right now... yeah.
Mostly... it's probably the last point. Things are looking pretty grim for THQ these days.