Seems insane. Surely every single company would fire you for doing that. If you put this in its proper term it is "corruption" and you should definitely worry about the legal implications of doing this.
Agreed. It's based on accepting personal payment to spend company time and resources doing something other than what the employer wants. Sounds like low-key bribery to me. I imagine they're aware of this. The FAQ section states that employees are kept anonymous, but also says they verify if it's really a Google employee by sending an email to their google.com email address, which Google can of course see.
If you're salary, then that doesn't enter into the equation.