In many cases, I'm not sure if it's a default position.
Of course there are people who don't follow politics or political philosophy well-enough to express themselves or are just not intelligent enough to do so. I presume that such people are the type that could end up abdicating their policy thinking to one of the political parties.
However, I haven't seen where these types generally have all that much to say. I'd guess the consequence of abdicating one's thinking is that you don't end up with many thoughts to post.
I've noticed that almost all posters, even the best ones, frequently offer superficial posts (at first, anyways). Oftentimes, it is only with a little prodding that more substantial discussions emerge (though a fair amount of the time, no one attempts any such prodding).
At first glance, a lot of thread comments may seem to be based solely on the stances of political parties, but for a great many of the posters, you can get some reasoned debate if you merely ask. Of course there are a few that have little reasoning, and, ultimately, it's my point that it is not merely agreement with some party's policy which makes for empty (worthless) contributions to the forum, but rather it is when someone is pressed but then display a lack of reasoning coupled with a willful blindness to their reasoning deficit which gives us worthless comments.