As for your being able to kid your fellow Italians in such a way, that's more an exception than anything else. Would you get offended if a non-Italian called you a wop, but didn't mean anything by it?
Maybe not, if you're as enlightened as you sound.
It's hard to explain why black culture is different. It has a lot to do with how we got here. Our ancestors were dragged here in chains. They were the property of people who went to war over the freedom to govern themselves. It was a crime to educate them or treat them with any semblance of dignity, the strong were prized over the smart, but it was perfectly okay to sleep with them. Then after they were freed, these uneducated, unaccepted, unloved people now had to fend for themselves, & try to survive while every day laws were being created to prevent them from having a real place in society. Manywere forced to go back to work in the same cotton field they'd just escaped. It took well over a century for open, regulated, government-sanctioned bigotry to be eliminated, & during that time the racists became more creative & clever. They found ways to make the legal system, the educational system & the economic system work for them surreptitiously, rather than blatantly.
Now we're a group that has gotten so accustomed to a certain kind of treatment, we expect it everywhere, from everyone & THE WORST PART-often, albeit unknowingly, we do it to ourselves.