Target Fired for doing something everyone else does Walled lake Michigan

 First off not sure why if you need more money and ended up getting a second job why you then in turn reduced your hours with Target..seems like one would cancel out the other.  Not only that but what in the world this has to do do with you getting fired. 

Also, if Target is like most other stores, if a store gives employees certain discounts or "perks" those perks are programmed into the register.  Such as employee discounts, I bet that there is a specific key you press for the employee discount and don't just make up some number to enter in.  In food I know some resturants will allow the employees to drink as much soda as they want, but the second they take any food without paying they would be fired.

Now, the only one who knows the entire story is you.  But if you were doing this and it is against policy(which I bet it was), regardless of how many people were doing it you should have realized it was wrong.  Perhaps it was okay to give "some" discounts but you took it to a new level and gave discounts way above and beyond what you should have done. 

Then of course there is another possibility, perhaps they noticed a shortage in the area and they sent someone in undercover(posing as a customer or perhaps even another employee) where you failed the test..but everyone else got smart and didn't go against the official policy when they were tested.