slapped....scratched....bit...whatever he feels like....
LOL! Why are they so ticklish on their bellies? Roxanne is the same. Sweet, but will retaliate if the tummy is touched.
The behaviouristic answer: the underside is the most vulnerable part. Your dogs show it to demonstrate their submission "Look, I'm totally helpless, I surrender myself to your mercy, you're the boss"
Cats show it when they feel relaxed, safe, at ease. They trust you when they show their belly. But they don't surrender, so when you dare to touch it, you breach the bond of trust. It's a transgession in their eyes, thus you get punished.
The real answer: they're evil and like to lure you by presenting the fluffy, cuddly belly.
But of course, they're all individuals. Some cats tolerate, even like it. Our Cäsar tolerates it from the girls, but not from Oliver or us adults.