Actually I believe it's done that way for liability purposes. If someone gets hurt trying to get 'into' the compound over that fence they are technically trespassing whereas if they get hurt from outward pointing barbwire they have not yet trespassed and can sue.

That's not to say that it wouldn't be effective for that purpose and in some cases possibly applicable but when you think about it there are just too many innocuous places protected by the same fence construction to conclude automatically it's the commonly held idea that it's to 'keep people in'.