If your executives start selling off shares before the employees who had been there waiting for their paper value to turn into real value get the same opportunity (as you said, with founders it depends on if its reasonable), then even if its legal its going to destroy any trust and the best people will stop believing in your company and move on to someone who hasn't demonstrated working against the employees interests.

In your last case, if someone wants to cash out equity but employees can't, then you've destroyed trust and your company may fail in the future because the employees can't trust you to protect their equity.