Two ways of looking at this.
Firstly when I go to work I complete a minimum of eight hours shift in the confines of an office block with very little opportunity to buy food. There is a single vending machine. I either have to bring my own lunch or pay for one from the vending machine. If I did not pay this would be theft.
There is little difference between this and the cabin crew story. If the cabin attendant knew he had to pay it seems he knowingly took something without following correct protocol.
On the other hand one could argue that this is Ryanair and he had limited options on what to eat, and had to consume something to ensure he was fit to complete duties. Maybe instant dismissal was a bit harsh and that a warning would have been more appropriate?