Well, I don't know all that much about it, but here's my understanding of the situation from what I've read:
-Defence of the Ancients is a WCIII custom map game type created by some dude who wished to remain anonymous, stopped developing and passed the torch on.
-He passed it on to some other guy and walked away
-The guy he passed it on to turned it into Dota: Allstars, the game it's known as today and eventually passed it onto another developer; Ice Frog. He then went and worked on League of Legends. Ice Frog seems to be attributed to the current success of the game. He's developed and maintained it for the majority of the lifespan of the game/custom map/mod/whatever. Basically, he's the curator of the game and the connection to the community.
-Valve hired him to make dota (I don't think this stands for "Defence of the Ancients". Probably to signify that the original, mod/open IP will remain out there, under someone else).
I could be wrong about it, but I don't really see anything unethical going on here. It's open and this is part of that, but from everything I've come across, the guy they hired (Ice Frog) is the man behind it. Sure, someone else actually created the concept and created the original mod, but they walked away and passed it on long ago. Valve approached the man behind the game and, from what I understand, not only involved him, but we can also assume compensated him very well for this.
If the creators of DotA are popping out of the woodwork now and calling foul, I feel very little sympathy for them. If you want to create an open ip but prevent others from capitalizing upon it, there are routes you can take to do so...they didn't, and I assume it isn't because of lack of knowledge. I assume it's because they felt that they had put in what they could to the project and passed it on.
As far as I know, the original creators haven't come out and spoken out against this. There was a trademark claim regarding the "Defensives of the Ancients" name a few months ago, but from what I've read, the person who actually put the claim in wasn't actually involved in the creation of the game at all but rather the administration of a fan-site which was the defacto official site of the mod (which will happen when shit is open). I think the "curator" between the game creator and Ice Frog also supported this...as he was involved in the creation of Leagues of Legends with the guy who ran the website.
I can see their point: you shouldn't be able to just waltz in and make a sequel to something like that. But, the reason you don't see this happening more often is because there are protections out there, and this is a very unique situation. They haven't done much different than they've done with DoD, CS, or L4D to some extent: They went out and acquired the developer behind the game. The difference here is that there really isn't an IP associated with the game to buy....or protect. Note that this isn't the case with the majority of open source projects (which still retain quite a bit of protection). The controversy I've come across surrounding this game, however, hasn't touched on this at all.
I'm not saying it's right, I'm just saying it's not all that wrong and not something you can easily liken to EA or Activision just ripping off a popular mod.