that kind of makes things difficult, as all possible scenarios can be imagined - point being, I do not feel it is right to say that HS should be used as an example of a place where the DLC can be distributed, because there are games which may have DLC's distributed through it at some point in the future. The examples you used do not exist (yet)... HS do not at present time distribute DLC's, nor is there any indication that they are going to do so.
Is is possible that they will do so at some point in the future? Off course there is. It is also possible that some point in the future HS will only provide Steam keys for all purchases...
Yes, but we're talking about whether or not it is a possibility or impossibility. A year ago, people would have told you that Humble Bundle will never ever ever
sell a DRM'd product or a non-indie title... they proved that wrong last November. At that same moment, most people would have told you they sell games, not ebooks or music (by itself). Yet they have done just that at some point.
We aren't talking about things that are far beyond plausibility here. A DRM-free DLC product is, in essence, the same as a DRM-free game... it's an install file. And Humble Bundle sells install files, ebooks and soundtracks at this point. I'm asking you to imagine that a DRM-free videogame storefront could start selling DRM-free DLC, not to imagine that GoG will suddenly become a postmodernist store catering to horse buggies and baby cannibalism products.
Not all imagined scenarios are equal to each other.
Is there maybe a reason why HS do not provide DLC's? probably. Can they be solved? Probably. Can all games in the world be distributed DRM free? Probably. Should we abolish wars and have equality between all men on earth? Probably. Idealism is good, can not really be used as examples in an argument.
(also, AFAIK HS do not sell Dungeon Defenders, there is only one build up there for those who got the bundle. This means that if there are any future DLC's (though I do not think so, I think the game has run it's course) it is not certain whether it will be given to them as it will require a completely new package in the current system)
I imagine that HS doesn't provide DLC because no one has sold DLC through it. I think that's the only reason it doesn't. I don't see any clues or claims in any interviews or even the FAQ that would lead us to believe anything beyond that.