That's really stretching the meaning of "virtually the same map". LoL and Dota 2 maps look similar on the minimap (HotS not so much), but they're drastically different when you start looking at points of interest and juke paths. In contrast, Dota 2 used to have almost exactly the same map as DotA (differences being shops and hero selection, which were changed from ingame units to actual UI menus).

Not to mention that Dota 2 also copied the exact skillsets of heroes and items. Not just likeness, but even most silly bugs that were due to the Warcraft engine.

Edit: Anyways, this is getting sidetracked. My point is that Warcraft 3 was lax with intellectual property for the longest time, and mods like DotA most certainly leveraged that.