It's not about cross-posting. It's about not letting all folks reading your cross-posted posts that the question was asked elsewhere. Without that knowledge folks might spend/waste a significant amount of time on your question when either an answer is already given or some comment was made that would given one insight as to how to solve the issue.
In short, you could be wasting time time of the experts from which you're asking for help. It's not polite at best.
