I'd guess that they'd want to give each employee a unique id, and they used the SSN because it's something that every employee has to have, so they didn't have

I'd guess that they'd want to give each employee a unique id, and they used the SSN because it's something that every employee has to have, so they didn't have to generate a new id number and sync it up with their accounting software. This has obvious security issues, as we see here, of course. They'd need it integrated per-employee with accounting, since it'd be tracking tips via credit cards which have to make it into payrolls and taxes etc.