IANAL, but I feel that coffee shop case would not be different if the software were under AGPL... if you are providing a service to other people, even if "the other people" are the customers to your shop, it could be argued that under the terms of the AGPL (not to be confused with GPL) they should have the right to see the source of the service that they are making use of.

But if the SOWPL requirement really does apply to private code that isn't providing service to others, the implication would be that even if you are the only user (no coffee shop customer), and even if you are the only one who knows about it, you would still need to make the source code public in some way... which I feel this is very impractical and probably unenforceable anyway.