You don't need a study to confirm this. Anyone with basic reasoning skills can see that a combination of low caps, high speeds and overage charges are just another revenue stream for an ISP. There's no financial incentive to upgrade network infrastructure if your existing structure can earn you tons of money in overage charges. -- "Net Neutrality" zealots - the people you can thank for your capped Internet service.
Since so few people hit the caps currently, I don't think it's good as an immediate source of income. I'd guess it's more to save on current infrastructure costs that would be needed for future growth.
I suppose it's possible that slowing progress in wired connections makes wireless more attractive, which can charge more for. Investments in lobbying and buying out wireless competitors seems like a better use of funds than going into FTTH and better backbone.