It's not an agreement, it's common sense (plus politeness). And the point is not to forbid crossposting, but to explicitly point out the cross-post:

If you crosspost on sites A and B, and you already get a good answer on site A, persons reading only site B will not be aware that you already received an answer on A, and will try to unnecessarily answer your question again. You, kind of, waste the effort of people trying to help you.