Except in this case its like calling your game "Deathmatch" or "Capture the Flag" or "Conquest"
Anyone stupid enough to try to trademark this would be shot down by dozens if not hundreds of other developers, and so will Valve in this case. Nobody has the right to the DOTA name, as its the name of a sub-genre more than it is the name of a specific game.
Valve simply trademarked shit that didn't belong to them from the get-go, it was an obvious troll aimed at Blizzard who has to sue, or they'd have to enforce valves copyright on starcraft/sc2/warcraft3 mods and maps. It'd be a clusterfuck of epicness all thanks to Valve being true dicks.