Can my employer enforce rules that have not been clearly stated or even written down?

Showing less favorable treatment to an employee is not illegal unless it constitutes some form of legally actionable discrimination. In other words, it must not be due to their race, religion, age (over 40), disability or any other "protectected class". Additionally, unless you have an employment contract or union agreement that prohibits your employer's actions, they are legal. The fact is that most employment is "at will", so a company can set the conditions of the workplace much as it sees fit. For your part, you can either put up with this unprofessionalism, continue to complain but risk termination, or quit. 

If you don't have a written employment contract preventing this (such as by setting forth a disciplinary process which must be followed), then you are an "employee at will." An employee at will may be disciplined--up to and including termination--at any time, for any reason, including unfair or even factually incorrect ones. Also an employer may harass or laugh at employees and may play favorites. An employee at will has essentially no rights in or to his or her job. So what you describe is unfair, but legal, and if you don't want to be in this environment, then you need to seek other employment, since you cannot change your employer or how they treat you.

If you don't have a written employment contract preventing this (such as by setting forth a disciplinary process which must be followed), then you are an "employee at will." An employee at will may be disciplined--up to and including termination--at any time, for any reason, including unfair or even factually incorrect ones. Also an employer may harass or laugh at employees and may play favorites. An employee at will has essentially no rights in or to his or her job. So what you describe is unfair, but legal, and if you don't want to be in this environment, then you need to seek other employment, since you cannot change your employer or how they treat you.