Stereotypes of any sort are harmful, as they assert generalizations over a population. Humans, being as widely variable as they are, routinely defy stereotypes, making them misleading. As for the double standard, I suppose that national stereotypes aren't as offensive because they are becoming less relevant. The internet and other media are quickly bringing the people of the world together and creating a common culture, so a person from another country is no longer as unsettling and alien as they once were. The different ethnicities, however, don't appear to be homogenizing at any appreciable rate, so two persons of different ethnicities will seem just as "other" to eachother as they would have centuries ago. I believe that racism (and also xenophobia) is an instinctive reaction; an evolutionary adaptation to ensure we would feel no remorse for wiping out competing tribes back in our hunter-gatherer days. Since the different races are still pretty much just as "different" as they've always been, racism is still a serious problem that must be carefully avoided and discouraged. Since there is a bit of global cultural homogenization going on, xenophobia, on the other hand, will likely continue to diminish over time.
Racial stereotypes belittle people on the basis of appearance. Satire is a mirror into the soul, i.e., it is deeper than any form of bias; bias is natural in human beings, it should be actively opposed.
There is a reason to every sterotype, some times they are unfair and/or unjustified, but there is a reason behind them all.
Ethnical(I will not use race when speaking of humanity) may be even more unfair than national because we each ethnic group has their own ones and we can't see the others reasoning for them. As a european the way other ethnicities are stereotyped is largely result of the colonnial age when europe was technologically more advanced than rest of the world, thus giving a feeling of superiority.
Then again I don't think skin color should count for jackshit, after all Italians are often labeled in "White" category... As Berlusconi said himself "He has a nice tan.", he spoke of Obama, but from a north mans POV, Berlusconi was the tanned one .
But stereotypes in general are fun to use, and there's allways a reason why that one exists.
i think the problem is how discriminated against black people were in the past, whereas the only person who has ever maybe discriminated famously against countries was John Lenon, who wanted to get rid of 'em all (i know, frontiers not countries, but you get my drift).
different countries have beaten each other up so much that there is no real discrimination against one country to another, except countries that really hate each other, and i think you'll find that if you made some jokes about some countries in some other countries, it would be considered unacceptable.
I think it depends on the scope. If we were to talk about someone sipping a 40 and sitting on their stoop blasting Fiddy Cent as they collect benefits, we'd be bitched at for saying "Black people" do this. If we said "hood rats" do this, the number of people getting upset practically vanishes. Why? Because we've differentiated the subset of people from the group as a whole. Try to describe white people in redneck/bogan/chav/ned terms sometime, and the same thing happens - they get butthurt and say only he hicks do that - if you highlight you're talking about rednecks/bogans/chavs/neds from the start, no one gets upset. Same with cholos vs. Latinos, ABCDs vs. Indians, ricers/AZNs vs. East Asians etc. As long as you indicate you are not talking about the group as a whole, no one gets upset.
Nationality stereotypes are more acceptable because they're on the whole less negative. They're usually playing on cultural quirks - English politeness, rigid German discipline, Russian/Ukranian stoicism, etc. However, there are plenty - PLENTY - of national stereotypes that are offensive. Talk about a stupid Pole, Irish drunkard, violent Scot, lazy Italian etc. and you're going to take heat just the same as you'd take heat for talking about the thieving Gypo.
It all depends on the intent and scope of the stereotype and whether it is intended to poke fun at its target or whether it's intended to denigrate its target.
Stereotypes are bad because they generalize an entire group of people based on one arbitrary thing or other. They are also self-perpetuating, because people tend to internalize stereotypes about themselves, even if they are negative ones, and start to behave in ways that are "expected" of them. Stereotypes aren't really true as such, they're mostly a product of prejudices and confirmation bias.