Attempting to answer this though I am a volunteer:

Basically, the Network feature is extra calculation and memcache (cached stuff making the site go fast) necessary to generate the posts on the page. It doesn't even work on my hack because I don't have memcache set up, for instance! The person who is paying for the account is paying us so we can provide it for them. I don't think OTHER people looking at it adds much to the cost, since it's already done for the paid user. The extra processing is why it's a paid feature, but there's not a huge hole in the dike really, or at least, I don't think it's much of a concern. (Official answer may differ.)