Permanent does not mean untouchable. He could fire you even after you got that meaningless moniker of "permanent."

MANY EMPLOYERS DO THIS TO AVOID PAYING HEALTH INSURANCE...

There are a variety of reasons this might have happened. Maybe your employer/boss is a jerk and it is their way of keeping costs down. If it frequently happens to others there then that’s the likely reason. On the other hand your employer/boss might not have been happy with your performance and felt it was better to fire you while you were still “temporary”. In the USA, in most states, employment is “at will” which means any employee, “permanent” or not can be fired for any reason or no reason at all. On the other hand, larger companies hiring professionals tend to be more concerned about potential law suits and “bad will” in the community if they appear to fire employees for potentially discriminatory reasons - Which is effectively the one exception to the “at will” employment rule. For that reason most either build a case documenting employee problems if that is the situation or fire the new employee during the initial probationary period.