Is this legal?

[FrIgHtNiGhT]

Alright, here it is: I recently started working for a dialysis contractor here in Michigan. One of the hospitals that my (former) employer serviced requested that all employees attend their hospital introduction/orientation, so I went. During the orientation, I answered a cell phone call and did not leave the conference room until the call was almost over. (The call lasted all of 56 seconds.) A day later, my employer was contacted by said hospital administrator and also by the CEO of said organization. (She was at the orientation.) I was not to return to said hospital because "my level of professionalism was not satisfactory." Answering a call in the middle of a meeting is: Rude. Yes, I can accept that. Unprofessional? Sure. (How many doctors have you seen take calls in the middle of their interactions with patients or with nurses, other staff members, etc.?) Is this a valid reason to be barred from employment at said facility? Maybe I'm a little bit more pissed off about this than I should be, but do I have any recourse here? (Yes, I understand the concept of at-will employment, but damn! I took a phone call! They forced everyone at the orientation to participate in a prayer at the beginning! Nobody's said anything about that except me!)