They say in Harlan County(from Bloody Harlan by Florence Reece)
There are no neutrals there.
You'll either be a union man
Or a thug for J. H. Blair.
One interesting--and irritating--feature of online argument, especially in an election year, is the routine assumption that everyone is on one side or the other and that which side you are on determines what you say. If you say something favorable about Governor Palin you must be a Republican supporter and are therefor obligated to respond to any argument offered against Senator McCain. If you say something favorable about Obama you must be a supporter of the Democrats and obliged to defend Obama against any and all arguments.
I am not supporting either ticket. When I commented on my blog that I thought Obama was the least bad candidate and offered some reasons why, with luck, he might actually do some good things, I promptly got labelled in a variety of places as having endorsed him. When I spent a number of blog posts here commenting on the election, some of them defending Governor Palin from blatantly dishonest quotations out of context, at least one blogger found it suspicious that I was saying so much in defense of the Republicans. In one recent online argument a poster simply refused to believe my statement that I didn't plan to vote for McCain. It was obvious to him that only someone committed to the other side could possibly question his side's version of the particular issue we were discussing.
In another argument on a different newsgroup, I pointed out that Biden had misrepresented the role of the Vice President as set out in the original Constitution. The poster who responded apparently thought I was defending Dick Cheney's claims as to where in the governmental structure he belonged and what privileges he got thereby. I have not actually paid much attention to that issue, and in any case the role of the VP may possibly have changed over the past two hundred plus years, especially after the Twelfth Amendment created our present system of electing a paired President and Vice President. Biden had cited Article I of the Constitution, Article I directly contradicted what Biden had said, and that told us something about Biden, whether or not it had any relevance to Cheney.
Part of the explanation of the pattern is, I think, the natural human tendency, probably hardwired, to view the world in terms of in group and out group, us and them. If I defend Palin or point out Biden's errors I am obviously not part of the Obama in group so must be on the other team.
There may be a second element. Most people are not very interested in political, economic, historical matters. But most people do enjoy cheering for their team. So political arguments, especially online during an election year, are populated by a lot of people who are arguing not because they are interested in the ideas but because it is a way of fighting for their side. It is natural enough for them to assume that everyone else is doing the same thing.