She may have just not wanted to ring herself up on the register. Looks kinda bad if you do that. Maybe she wanted to wait until another cashier was available to take her money so she wouldn't get fired for theft.
That didn't work out so well, but I could understand the mentality.
In a retail position, you're generally not allowed to ring yourself up.
While I am generally against "no tolerance" policies, and I am in this case, as well, I can see where the store has a valid point. It isn't just that she stole, she also ate while at her post in plain view of customers not during her regularly scheduled break times.
I don't agree with firing her, but she did put the store in an ackward position. On one hand, common sense dictates that firing her is going overboard. On the other hand, she could have established a president that it's OK for store employees to munch on whatever they want whenever they want and "pay for it later."
What she should have done is called someone over to releive her so she could go to the break room and drink some soda/juice.
Honestly, her story is just a little too convinient. She probably does this regularly and management simply had enough of it. The longer someone works at a place, the more they treat it like home. She got caught and now is facing the music.