And even if it isnt part of the secret shoppers thing, it's courteous to ask if they need shoes, because, well, they might need shoes.  There's a guy that bowls tuesday nights where I work, decent bowler, brings usually four balls with him, uses house shoes because he doesn't own his own.  I guess you can never know, and it's better off being polite and asking.  I'm pretty sure it's not meant to be offensive if someone asks if you need shoes or not.  I guess it does go along with the "would you like fries with that" thing, because if you didnt ask if they wanted fries or not, and didnt give them any fries even though they wanted it but didnt say it because it was expected they were going to get fries with it, I'm sure they'd be pretty pissed off when they didnt get their fries.  So, I guess it could be for reassurance, as well.