Obviously the people who originally created the MOD should get some credit. But when has that EVER happened in the history of our beloved hobby?? Valve has a long history of making games based on mods. They usually hire the team involved in the original MOD as well. I dont know about DOTA, but if Blizzard wanted to get on board with a standalone DOTA game they should have done so before Valve did. Right now they simply look like douchebags trying to protect an IP out of self-interest.
If on the other hand they developed their own version of a DOTA game, that would be a different matter. As far as I know they arent doing so.