"ISPs get paid for how much data the consumer uses."
Precisely, they are paid by the consumer, and does the consumer really want to pay the ISP) just so that they can receive ad's that only benefit a third-party, particularly when they know that their broadband subscription isn't being subsidised by the ad revenue?
"They don't really care how you use up that allowance you've paid for, they profit by you using more of it."
Only to a certain extent. ISP's do care about how (and when) you use your allowance because services such as telephony and streaming media/tv have a different traffic profile to email and web browsing, which have an infrastructure and service management impact.
As for profiting from usage, that only really applies when the customer is charged per MB, whilst this is the case for some ISPs (and the majority of mobile telcos), it isn't necessarily the case for many fixed line ISPs. However, here in the UK people may have missed the reasoning behind Three discontinuing their unlimited mobile data tariff, namely, because of market development (ie. more subscribers and greater usage of mobile data) they no longer have an under utilised (3G) network.
So it is natural for ISP's to look at the ad networks and say, if you want to make use of our distribution and delivery network, then you need to pay.