To me, rumors (those in the Rumors & Events) should be done by the DM(s) for several reasons, and the fact that Bjorn has moved every rumour not created by DM's or which is in response to rumours created by the DM's in this sub-forum should help cement that fact.
Posting your own rumors, would in my eyes constitute as PR in some manner.
You either want to discredit someone, or blow your own trumpet... Either way, this is either a decree, a grant, espionage or the likes, and should, if it works to your benefit, be posted by the DM's or with the DM's permission, as very few actions are automatic.
If you, as a player/domain wish to instigate rumours, there is (usually) a point to it (and if there isn't, paranoid players like myself will still ponder why this news origin from this player instead of the DM).
To me, the rumor/event board is the tool which DM's use to keep people up to date and interested regarding current affairs... but where we as players have no immediate say. We may react, sure, but not instantly and besides the rumors could be false.
Allowing players to create their own rumors: say for example "troops are marching through province X towards Y" will get someone paranoid (like me) to jump at shadows, possibly activating troops I would otherwise have garrisoned, redo my DO etc. and in effect waste a lot of (IRL and IC) time and effort on nothing.
The word of Bjorn is the law... we (I) therefore take rumors posted by him seriously. They represent stuff that is actually known by enough, that is in fact a rumor. I can rely on him to be unbiased and post both that which I perhaps wanted (or used actions on), and that I'd rather hide. I can also rely on the rumors to be warped to the point where a feather becomes five hens, but that is another matter.
I cannot, however, ask the player of a domain to post news from his own territory and expect it to be totally unbiased. Some may be able to do so, but for the most part I would believe that some manner of self-censorship would enter the picture.
Honestly, I know very few people who are masochists enough to initiate a devious strategic move, and then afterwards post rumors of said move for all to see...
But that is just my PoV.
Edit:
I realize that I may be an extremist here, nudged severely by my current studies as a budding journalist. But I'll still claim that there is no such thing as negative stories from your own interest group unless someone rats on former employees.