Anecdote to add, I've never across 20 years experience (and many jobs) not had a moonlighting clause in my employment agreement.

As someone who hires, I'd be worried about the IP implications of even suggesting I pay them for outside work prior to them leaving their prior commitments.

Huh, interesting. I think I'd just walk if presented with a firm one. I constantly have other things going on, even when FT employed, and honestly no employer is going to pay me enough to constrain my options.

I assumed it was boilerplate. For the record I've also never had an employer have a problem when I notified them of outside work.

That might be different if the outside work in question was an interview for a new jib though...