Just a guess, but I'm wondering if it's more about their system detecting someone serving almost entirely image requests, versus someone serving 20-40 images per web page, a pattern more consistent with images served alongside a web page.
It also has to reach a critical point. Serving maybe 100k unique visitors and 1tb in a month isn't that big of a deal, but 100x that might be enough to where you're using a lot more than every other free customer.
