Can my job sue me for adding discounts?

Up until a few weeks ago, I was a server at a restaurant. I was terminated for adding a 10% discount onto guests' checks after I had authorized their cards in order to give myself a better tip. The total of the customer's check remained the same, the only difference is that it was more money in my own pocket. Upon termination, they told me that each time I did that was a count of fraud which, if I had known, I never would have done it. Now I have heard that they plan on pressing felony charges. Can they do this? What kind of repercussions will I be facing? And what counter measures can I take? If it matters, I am not the only person who has done this. 23 other (now former) employees have been let go for the same reason.