As for whether you could prosecute without the criteria you listed, I could and in effect have. However it is the prosecutor's job to decide whether their is sufficient evidence, and then to prove it in court, the citizen is supposed to report the crime. Without trotting out all the language it isn't necessary to leave the store (helps though at trial).

Considering the story, it isn't unreasonable to surmise one of two possible scenarios, the store has a policy (most convenience stores are such nickel and dime businesses, they watch everything)or this customer (who had done it before) was told that if he did it again the police would be called. Charging him for the second dog is a good idea.

I do agree that training is important, but I am sure you would never train an employee to act unlawfully. Besides my guess is if it was a new employee, you would sit down and 'train' them to avoid a similar situation in the future, and if a long term employee you are the loyal type, so a word maybe, but fire I doubt it.

A couple of years ago I did force feed a fired employee back down the throat of a fast food owner. But it was a little more complicated than the above scenario.