> why would a company pay them anyway?

Because we have laws to protect workers. You can't just not pay an employee and call them a volunteer as a for profit company.

There a legal limitations on what a for profit company can do with volunteer labor but companies have been increasingly bold about pushing those limits.

Often times it is hard to tell if a program violates those limits since particpants are put under strict NDAs.

That sounds like a definitional problem though. If a person is choosing to volunteer are they really an employee?

I general don't agree with NDAs though, so on that front we definitely agree there's an issue there. Especially for a volunteer, I would never sign an NDA when I'm volunteering my time and effort.