I don't think their goal is to attack things that the government is perceived as being responsible for protecting. So, mall shooting is fine because mall security is the mall's problem. Airport security the government is responsible for.

So why school shooting then? The goal with those, to the extent the government is involved, is to drive outrage to get guns banned or restricted. But I actually think we have a lot of mentally ill kids with crappy parents, so most of those shootings are just the result of that, just like the flash mob store robberies and big fights of kids.