Interesting point. The reason for this I imagine is that people love to argue and they love it when their view points are backed up. If you read an article and it corresponds with your views, you're more likely to upvote it. Especially as it means it will be seen by more people and your beliefs must win!
With an informative article however it's far easier to forget to upvote. I imagine people see it as being less important.
Just my $0.02 :-)