|laziness, impatience, and hubris|
Although I have never done a thorough search, I don't remember ever seeing an official PerlMonks policy on this. Also, every time that I have seen folks "complain" about someone cross-posting and not saying that they cross-posted, I don't remember seeing any of them point to an official policy on this.
What follows is my personal opinion on the topic:
On one hand I can understand the rationale behind those that want folks who cross-post to share the information about the cross-posting. For those who frequent multiple sites/mailing list/etc., I can also understand how helpful and beneficial this can be for them.
However, what really drives me nuts are those individuals who get down right nasty with anyone who cross-post without mentioning anything cross-posting. Of course, these individuals also fail to point the cross-poster to the official PerlMonks policy about cross-posting.
In my opinion, that's the kind of behavior that can drive users (especially those new to using Perl) away from this site and possibly the Perl community in general. Instead, I personally would prefer that these individuals make their point/request in a more respectful manner, which I think is a more effective way to get others to agree with your view. What's that old saying? You attract more flies with honey than vinegar?
For those that just loathe cross-posters who don't say anything about cross-posting and choose to respond angrily and harshly, perhaps they should instead focus their efforts to appeal to the gods about creating a new official policy on cross-posting. Then they can refer folks to that policy.
Well, I guess I'll get off my soapbox here and wait to see how low this node gets downvoted. :D