Imagine if Google adopted a policy where each user was only allowed 300 queries a month. That means Google wouldn't have to expand their infrastructure to satisfy a growing demand for information. The only reason they don't do this is because Google profits from each query.
ISPs don't profit from their consumers downloading more data, it only costs them building more infrasture and employing network admins to maintain them. Its just internet providers generate a ridiculous return in profit for the amount of money they actually invest to provide internet. ISPs don't need to compete with other ISPs because how profitable internet is for them. They need incentive to provide better service and until something like Google Fiber forces disruption in the market; engineers will have an artificial roadblock preventing them from innovating.
For example a cloud based OS where users can access their PCs at home in realtime from terminals abroad will never be a reality until America is upgraded to fiber optic bandwidth. It causes to much latency to download and mount your filesystem onto a local machine. Anybody with experience using Plan 9 remotely understands this problem and knows I'm right.
As we become more dependent on the cloud to sync data between our mobile devices and PCs, datacaps will stifle innovative growth. Its something we need to vehemently battle to shift the market in our favor.