when dota 2 got announced, pendragon or one of the other old dota developers who went to riot copyrighted DotA and defense of the ancients. they then gave this to Blizzard (probably hoping Blizzard would use their legal power to shut them down).
this is why Valve only call it Dota (no capital A) and never ever reference it as defense of the ancients. theres alot of shit flung in the background but Blizzard copped ALOT of bad press last time they challenged valve over this issue, i seriously doubt they will do it again.
copyright/trademark still stands. you can't really copyright similar apperance. but hero names have been changed, centaur warchief is centaur warrunner. obsidian destroyer is called outworld destroyer. it's enough that they can't be sued.