>They certainly can offer flexible hours to full time employees - no where does it say "Americans shalt work from nine until five so sayeth we the founders".

I don't think you understand how a business like this works. For a customer serving business, they need to make sure they are staffed appropriately.

Would it make sense for a restaurant to offer all of their staffs "flexible hours"? Is it ok for the waiter to show up at 3am when he has trouble sleeping and just get paid doing nothing?

It's not about 9-5, but it's about when the customers show up.