They could attack the bushes--it's more a matter of programming the AI to know what bushes to attack and when. I've experimented with this capability, and in maps with lots of destructible objects, it slows the Computer's turn down dramatically to have consider every last bush and tree as a viable attack target.
To get around this, I hand-mark certain destructible objects as targets (and the game automatically marks player-created destructible objects as targets). The unpredictability of the layout on procedurally generated maps makes this technique impossible to utilize, however.