We've all seen the videos of them destroying temples and other religious structures, but what was there about a theater that made them decide they "had" to destroy it?
Archaeological tourism is big business. In the case of future Palmyra possibly the only potential tourism business for the foreseeable future.
Therefore destroy the theatre and any and all other archaeological remains and you debase if not destroy the economic future and potential of the region .... that is the
ISIS philosophy, indeed ideology in action...... return the economy and with it the society of the region to to the seventh century, the time of Muhammad, if not the
stone age. Remember these people aren't builders, they are cancerous destroyers and destruction is at the core of their ideology.