The police cant arrest, as in take to jail, people for misdemeanors not committed in their presence.... DUI being the only exception. Even if you or your staff had stopped the suspect 'red handed', it would be one of you making a citizens arrest.
So the officer did the next best thing, took a report and issued him a misdemeanor citation, for which he'll go to court and be held accountable. If hes got priors or is on probation / parole he could be screwed.
My dept wouldn't bother with $50 capers generally, we gave out a carbon copy "log report" to document the loss, and that's it.
It could have been a felony if:
The officer could have shown suspect entered your store with $0.00, the value was $400+ (some states vary), or the suspect had at least three prior petty theft convictions.
If the suspect had numerous brand new items on him and no receipt, a creative officer could have taken the guy for felony receiving / possession of stolen property... then been laughed at for making a $50 felony arrest.
Edit: BTW for a trespass, the suspect needs to be told to leave, then refuse... or be advised that hes PNG, then returns. If no police officer is around to witness either of the above, it's you or a staff member that witnesses the misdemeanor and makes a citizens arrest.