I can't speak for everyone of course, but for the companies that I have worked for, where I held equity, and the company was publicly traded, there were some strict controls over when and how employees could buy or sell shares, and three of them forbade any sort of option trading on the company stock. When being acquired there were even more restrictions on the stock that I received in exchange for my 'founders stock' in the original company.

Or did you have a specific scenario in mind?