Lots of guys went on AD, and stayed on AD, during the recent war years. Or they were furloughed and went on AD and finished their twenty years.
How do you define what is or isn't ok? If they're dodging billets it's okay? Dropping bombs on bad guys is okay? But a staff billet isn't?
The law allows it. The companies hire knowing the law. It's a plus in some circumstances, less so in others circumstances. I know guys activated right now. My guess is they'd prefer to be home with their young kids for Christmas vs sleeping in a tent overseas.