I got a written warning for using my employer's email system to send out a message to union members about an upcoming event. (A group of us are going to represent the union at an Earth Day festival, and one of the activities is going to be a rally against corporate practices that harm the environment.) My manager told me that I was being disciplined for using the company's email system for personal reasons or to solicit employees. But everyone sends personal email and email relating to The United Way, the Girl Scout cookie drive, and countless other causes, and the company just looks the other way. As far as I know, I'm the only employee who's been disciplined for breaking this rule; is it legal to selectively enforce company policies this way?