Part of the issue is that "Polite" in american has trended towards indirection. "I'm not sure that was the best choice" is American for "That absolutely was not the best choice and must be reverted immediately" .
A) That's not an American thing. US confrontation etiquette is still far more direct than British, for example, even if less direct than in, say, scandanavian countries. B) If the message ends up being the same, what is the actual negative effect? I'm not saying you have to like it, but it seems to be an entirely aesthetic "get off my lawn" sort of complaint. Giving shitty, incomplete feedback is just as possible using direct language. C) Since we're talking anecdotally, the people I've met that gave great, frank, direct feedback just did it because they out obligated to, and even in art school with young students, I've seen few people get defensive. It was calm, well-reasoned, and completely no bullshit. Then there are people who might have said very little, or maybe even a lot, but did it in a shitty arrogant tone and that got everybody's hackles up. Everyone I've met aggrieved about not feeling free to give direct feedback was the second type. C) Directness is just a slice of a much larger topic.