Wrongly accused at work?

Ok, i work for a store, we sell Communication items such as cell phones ect... Well i work by myself alot, my boss/manager frequently leaves me at the store for days on end. Well, last Sunday i worked alone, and Monday when i came to work, my boss was upset with me because he said someone shoplifted Sunday while i was working. My store has security cameras that are not functioning, but look like they are. Well the items stolen amounted to about 50 dollars in accessories. My Manager starts telling me i have to pay for the items which were stolen! And i wasnt even aware they were stolen. He called our district manager, and the DM told him to give me a written warning, if it happens 3 times you get fired! I feel this is ridiculous because it's impossible for me to watch the entire store, while doing contracts and informing customers interested in our carrier. Is this fair, it is impossible for me because some times there are 20 people in the store, and iam the only employee!