They’re betting that you will.
I’ve always suspected that opinion magazines would have a particularly hard time charging for content. Larry the Liberal may be interested in what conservatives think and read therefore NRO, but he might feel bad about giving us his money. (On the other hand, if you’re so inclined, click here.) Free online content is useful, I think, because it encourages each side to learn more about what the other side is thinking, which is a good thing — especially for conservatives, since we have better arguments.