Why does it even have SSNs?

Maybe I'm just showing my naivety of the US Social Security Number system, but why does Radio Shack even *have* customers' social security numbers?

I know the US SSN isn't directly comparable to the UK National Insurance number, but I'd never dream of having to enter that when creating an account (electronic or otherwise) with a retailer. Maybe it's more akin to giving my driving license number - even more bizarre!

Of course, there's always the further question of why it kept credit card numbers in cleartext as well, but hey ;-)