In certain circumstances it's considered bad netiquette to post the same thing in more than one place. It's incorrect to say "It is bad netiquette to post multiple copies of the same thread." because it's not necessarily true.

If it's against this site's rules, then fair enough. If it's not, then please don't attempt to push your point of view as though it should be everyone else's by using loaded phrases such as "Please don't justify cross-posting.", "It is bad netiquette to post multiple copies of the same thread." and "we are confronted with four identical threads".