We saw the same thing over "The Sopranos" if I remember.
Like it or not Italy has produced one of the largest, most enduring, and most dangerous series of organized crime syndicates in history. Syndicates which define themselves ethnically and still exist and are very powerful today, with organized crime busts making the news.
Back at the time that "Mafia 2" was set you didn't have a lot of organized crime in the US. The closest thing we had were large Irish gangs that did pretty much the same things, and the Italians killed most of them off in some of the first crime wars the US had ever seen.
Later on of course we did see other ethnic and cultural syndicates develop and have a foothold, and of course games set in more modern times do account for this.
It arguably wasn't until the civil liberties victories of the 1960s and 1970s that you really started seeing other groups of criminals developing in a big way. Sort of ironic, on a lot of levels, and a demonstration of how even the most idealistic efforts can cut both ways.
The bottom line is that "Mafia 2" is absolutly fine as period fantasy.
I'll also say that I think this case here demonstrates exactly why we need to seriously take efforts to curtail "political correctness" in general and stomp on those who want to try and find ways to enforce a ban on "hate speech". Just because something is unpleasant and might fit certain definitions of racism and bigotry does not mean that it's untrue. You start supressing or worse yet re-inventing history so groups like Italians won't be offended by things like The Mafia, and it also prevents people from learning from history and why things have developed in specific ways. A lot of laws and developments in law enforcement exist specifically because of the activities of organized crime, especially that involving tight ethnic and cultural groups that are very, very difficult for outsiders to infiltrate as a result.
On a note that is not directly related, I will point a finger at the politics and political correctness involving native americans, which has lead directly into historical reinventionism. While many get into the whole "good, one with nature Indians" and "evil, greedy white men" thing, it's not and never has been that simple. The way that things are taught now prevent people from actually learning anything about the period. What's more while something of a minority there are a decent number of natives who also get irritated because in an effort to make tribes look more advanced (or noble-primitive) and morally correct than they ever were, a lot of the actual culture and history is lost. A good example of this is how if you look at the tribes down here in the East Coast, you'll find that they weren't as migrant as tribes elsewhere and lived in longhouses and other permanant and semi-permanant structures, they also farmed. People like to portray the situation as the first settlers coming in ans screwing people with no concept of land ownership, that's hardly the case, at least on the East Coast they very much did understand the concept for example, and by portraying things differantly people tend to miss out on how the settlements were, how they farmed, and other details. On some levels this does make them more advanced than a lot of portrayals, but at the same time it also means that when they sold land they did so believing they got a good deal. People criticize settlers and such for trading with cheap baubles, but there were cheap baubles in proportion to the society of the settlers, not in proportion to the natives who couldn't make such things. If a tribe walks off with a couple boxes of costume jewelery in exchange for farm land, and they were happy with the deal (since they
cut it), it's not right to retroactively make judgements, especially seeing as they very much did know what they were selling.