It's because of the ratio of reads to writes. For many applications, especially in social/content, the creation traffic is an order of magnitude or two less than the consumption traffic. So free inbound bandwidth is a bit of a rounding error.
Here's a somewhat old article that captures the rule.
Goto facebook, or SO, or HN or youtube or almost any site...open firebug or whatever, and calculate the amount of inbound traffic to the outbound traffic (keep in mind that this is reverse for you and the host). You should notice that your outbound traffic (their inbound), which is mostly short GET headers, is like 1% of your inbound (their outbound).
Sites where you upload a lot of content, like YouTube, still have much, much more reads (outbound) than writes (inbound). Probably by an order of magnitude.
Systems like Dropbox would benefit - though they were already getting different pricing I'm sure. But this is really a tiny fraction of how most online systems work.
