'Because people metagame against them and team up on them merely because they are good... '
And this is entirely fair meta-gaming as far as i'm concerned. If these players lost a number of games they would go on to have less points and be seen as less of a threat. Anon gives them a measure of protection and thus makes their games easier (this is not as extreme as playing against yourself, but as always making the game easier is no fun for me - fortunately i don't think anyone's considered me much of a threat, but in the hypothetical situation...)
'I think the founder would argue that in its purest form, each game of Diplomacy would start completely fresh, so that players knew nothing about the other's style, trustworthiness, etc. Do you agree? '
I do not agree - i believe that diplomacy is inherently a social game, and previous social interactions(ie:reputation) are an important part of our decision making (and an important part of FtF games)
That said, the creator of diplomacy or even the creator of webdiplomacy is not a perfect uber-being who defines how we should absolutely play the game.(appeal to authority fallacy)
In fact, i think KNOWING someone's reputation forces players to build a reputation. Usually the best would be 'unpredictable style' OR 'varying trustworthyness'.
This may make the game harder, but i've no reason to be against that.
'I believe that they feel that players should approach each game freshly, with an open mind to new alliances. That, one can do, even with history in mind.' - +1
'but if you play at tournaments or whatnot, it's more like anonymous' - I guess, but if you can do some research online when you see who entered the tournament you may be able to find out a bit about a person's history...
and if they have no history of playing in tournaments then assume they are a noob.
Are there any tournaments which try to exclude people from knowing who won? ie anon tournaments...
My solution to the points loss/ganking the big guy problem is to let them sell their points (at least all but their last 100

)!
yes, let players pay to play in more games... it doesn't subvert the purpose of excluding people from playing if they're not serious, it just gives them an opportunity to prove they are serious...
(all imho, fyi)