It is probably due to the bug where AI will build from any buildings it managed to place, whether it be base structures, defenses... heck, even grids... and this placement would still occur even if you destroy the offending buildings (I think?). The AI in Single Player will rarely, if ever build stuff without direction while Skirmish AI in standard, challenge and co-op will do whatever suits them.
I'm pretty sure you don't want to have to deal with Infernos, Psychic Towers, Gattling Guns and Antares spawning inside your base....
That's the Risen Inferno support power instead of the buildable defense.
The AI is not programmed to use it in Skirmish, that is all. (Though this will most likely be changed in the incoming patch)