According to the FRCS native outsiders can be banished because it just removed them from the plane they are on, it doesn't neccessarily send them to thier home plane, just off of yours.
Someone in my group said he saw, something contrary to that in a Dragon, magazines Sage Advice, but since its is spelled out in FRCS and the Sage can't errata, I'd say this is correct (unless I missed a FRCS errata).