Yes, you can find WSJ readers elsewhere on the web reading free content, but there are few places where such an audience is highly concentrated. Behavioral targeting networks, for example, aim to target niche audiences wherever they go on the web, but many premium brand advertisers remain uncomfortable with the idea of dispersing their ads across a network. It’s much more comfortable, i.e. much more like traditional advertising, to reach a large percentage of your target audience in one place.
The advantage of the web is supposed to be that it’s more efficient than mass media advertising, i.e. reach only the people you want to reach. That’s why search advertising is so powerful.