That said, they are selling a service, and you're paying for it. Quite a bit for it. It would be pretty stupid for AT&T to use your DNS data and risk your $150/month cable, phone, internet subscription for an extra buck or two.

But with Google, you just don't know and their entire business model is predicated on selling your data. They are almost certainly using their DNS servers for some data-based operation.

And a last quasi-technical point... I'm sure AT&T and Comcast have good engineers on staff, but I'm even more sure that Google has better ones. I am less concerned about AT&T and Comcast because I honestly don't think they have the wherewithal and talent to come up with ways to monetize DNS. I'm pretty sure Google could.

And to your point, even if Google is giving away DNS services to only fight bots and measure internet speeds, they should at least say that in their privacy policy. They don't. They just say they keep detailed data temporarily and anonymized data long term. I don't use Google's DNS because I really have no clue what they're doing with it. In contrast, if my ISP does something, I can always try to sue them, or if that fails cancel my service.