1) Their culture is ignorant;
2) Their culture is immoral.
First, ignorance. This applies mostly to indigious/tribal cultures. For example, many of them die of diseases that are easily curable in the modern world. Should we let them die when we can easily solve the problem with a vaccination or some medicine?
Secondly, immorality. Places like Iran are full of human rights abuses such as oppression of women and execution of homosexuals. Can we let them perpatrate this injustice merely because it is done in the name of their particular cultural beliefs? Why should these people suffer because they were born in the wrong country? One could even be so bold to call our unwillingness to help these people a form of racism: you were born in a bad country, get used to it.
I know that there are many people who think that all cultures are equal ie cultural relativism, the rallying cry of the pomos. But for me this simply isn't true. There are things we recognise as inherently bad. So why do people do them, the reply goes. Perhaps because of ethnocentrism--people in your in-group are fine; people in your out group are just another thing to be exploited.
So if people don't have a right to do these things, how do we stop them? It's difficult. Is dialogue possible? Who knows. War? Probably be more of a disaster than Iraq. I don't really know what the answers are, to be honest.