The only reason the annual rate is so horrendous is that it is an annualised rate for a loan that is short term in nature. It's a "payday" loan, and if the loan of 275 quid gets you out of the mire for a cost of 75 quid, then to some, that's worth it.
Unfortunately, the services are prime pickings for these types of loans, as the services (and particularly the RAF) don't pay very well until you climb the pole a bit. If they showed three drunken SACs borrowing the money to go on the piss again, it doesn't have as much appeal as a nice officer and a bird. They don't show chavs in adverts for Corsas or little Peugeots, but that's who drives them.
I concur with Wensleydale. Financial worries lead to susceptibility to blackmail, and therefore a security issue. I knew plenty of chaps in the Royal Signals who lost their vetting because of these kind of issues. Perhaps the Corps is particularly susceptible to it, since nearly every job requires positive vetting, but I'd think the RAF is not far behind.