For the record, I think the idea is that they actually ARE a reasonably secure facility, by normal standards - you certainly hear enough about fancy new security procedures they're trying out - it's just that they happen to deal with insane geniuses like the Joker and company, for whom 'reasonably secure' translates to 'revolving door'. It's not about how incompetent THEY are, it's about how the Joker manages to escape on a regular basis DESPITE their relative competence.