As with accidentally mining into an underground base, I would say you causing a creeper to blow something up is still your responsibility as if you broke the blocks yourself. It's like tnt, you did not blow up the blocks, but you caused them to explode by lighting the tnt.
You can break the wall open into an enemy place fine though, as long as you replace the blocks, so there is no point to using a creeper unless you really want to, in which case the blocks would need to be replaced I am sure.
Really its up to staff still on this, but it seems like common sense to me. So there is my answer, but I wouldn't take it as a rule, and wait for staff to reply eventually instead.
