In an "at will" employment relationship, a company can set the conditions of employment much as it sees fit. This includes why to place an employee on an "improvement plan". That is so long as this treatment does not constitute some form of legally actionable discrimination (which it does not appear to based on the limited facts given) or if it violates the terms of an employment contract or union agreement. That having been said, you may want to consult an employment law attorney directly about this. Ater hearing all of the details of your situation, they can best advise you further.