Well...A quick reply...
A developer wants all users of his application to get updates as soon as possible, so he creates a Twitter channel for it. For him, it doesn't look like cheating.
A human develops interesting application for himself and wants to show it to his friends. Mentioning that it takes part in contest and inviting them to vote doesn't resemble a nefarious action.
In fact, I suppose that others also mentioned their applications to friends: it just happened that either quantity of possible users was smaller or the application was less enchanting or the developer was taciturn...

Yes, the rules could have specified that each voter should have used the application he votes for on an N900. But they haven't.
Yes, the rules could have specified that each voter should have registered some time before competition or posted not less than 5/10 messages or received not less that 2/5 thanks. But they haven't.
Yes, the rules could have specified that each link to the competition from an external website (like Twitter, Facebook, Wikipedia) should not be part of advertisement like "vote for this application here" but should be completely neutral. But they haven't.

Yes, the contest should have been better advertised. The maemo.org homepage could have included large headline linking to it, for instance. But it wasn't.

And guess what? I'm sure there were some cases when developer of one of competing apps gave link to the contest, and the reader of the link registered and voted for app of another developer.

"How can a user vote for a program if he hasn't tested it?"
By screenshots. By descriptions of other users. Imagine your N900 is broken or hasn't yet arrived from the shop or isn't bought yet. But yes, it's a shadowy ground.

"and to threat like this is just low, very low."
It's not a threat, it's a response of grieved and angered person. Imagine: you developed a brilliant application; you won trip to Conference; you haven't cheated at all to win; and then it's said there is a possibility you are disqualified!

"I think using friends and social networks can't be exactly called cheating, but its unfair and should be forbidden."
Well, where would the votes come from? The thread would be visited mostly by developers of site talk.maemo.org, not by usual users of N900.

"lot of developers didn't know about the contest."
Sad, sad...

"lot of users didn't know about this contest!"

Well, not all users of talk.maemo.org visit
OS / Platform > Development >
frequently enough to see it.
And not all users of N900 have visited talk.maemo.org

Before next competition administrator could send PMs to all users of talk.maemo.org.

"The rules were not perfect but, like kojacker said many times, any mistakes on this competition will help make the next one even better!"

I concur.