It takes two to argue, so you can't put it all on me if I respond to your post. You just responded to my post. We're debating an issue, and that happens on the boards and doesn't have to be anything nasty.
Disney has a standard way of handling this. As long as the restaurant is open, they accept the guest who is 15-20 minutes late. This has been said to me explicitly by a Disney dining rep, and has also been my actual experience and the experience of several other posters. All of that is since the $10 fee was instituted, not before.
The $10 fee has nothing to do with people being late- it's to discourage no shows & the last-minute cancellations that used to be chronic back when some people would schedule simultaneous ADRs for every park, so they could decide at the last minute where they wanted to go that day, yet still have an ADR waiting for them.