ISPs want to sell advertising, or data to advertisers. Why bother trying to advertise to a few geeks who are probably running PiHole anyway? Especially since doing that multiplies the hardware requirements 100 fold.

Possibly because the people who you can sell that data to are prepared to pay way more per "product" than the people trying to sell you fast moving consumer goods or ICO de jour...

Wouldn't surprise me at all to find there's a market where intelligence services can purchase lists of TOR users - for example...

True, although what are they going to do with the data? If it’s primarily for selling to ad companies, a tiny slice of privacy minded people aren’t worth much.

Err, snooping on a teen visiting "naughty" sites isn't exactly interesting from any perspective.