It's a very old strain of left-wing thought that leads to this attitude. Have you ever heard of
noble savages? The left tends to apply this romantic lens to every non-European culture. Everything evil about such a culture is always seen as somehow being brought out by the White man's carelessness or misunderstanding of it, or spoiling their good nature somehow.
Muslims are today's noble savages, and thus above reproach. Everything they do is excused and protected as part of their culture. Sadly, this was applied to even cannibals and tribes guilty of human sacrifice back in the day, so this madness knows no bounds. It's just that in the old days, average people were too smart to listen to that form of left-wing thought seriously. Now, there are enough of these starry-eyed, noble savage praising romantics running around that we have to worry what they think.
They should know better and stick to principles, but their hearts get the better of them and they start accepting sloppy ideas like thinking women shouldn't report rapes because it makes Muslims look bad, even saying that they were "tempting" them with their skimpy clothing and should respect their culture by covering up. The fact that few feminists are willing to attack those people for talking like that and try to come up with morally relativist arguments underscores just how much influence these people have.
These noble savage promoters are actually undermining people within these cultures who see these problems as problems, and want to reform it, by saying that they shouldn't be brainwashed into reforming and thus "spoiling" their beautiful and exotic culture with European standards and that it's perfect the way it is.
The problem with this kind of moral relativism is that it doesn't allow for people on the left to have any principles at all where other cultures are concerned. It's broken, and it forces people to look to the right for principles or values of any kind beyond the moral relativism accepted by the left. Something they're now desperately hungry for.