That is because of the long history of Russian imperialism is it not? Russia pushed out most of the native population and built a naval base. Personally imperialism, colonialism, and ethnic cleansing are not really strong moral arguments why Crimea should be part of Russia and not Ukraine. Also how did Russia view parts of itself that did not want to belong like Chechnya? Did Russia say "Hey, be independent", "Hey, join a neighboring country no problem" "Hey have a referendum and we'll honor it". We all know the answer, they started killing until they got what they wanted. I'm not seeing a strong moral case for Russia to keep Crimea, it seems simply that might is right.