It's sort of an absurd question, the prevalence of such things by ISPs was the literal impetus for DoH and DoT.
https://shkspr.mobi/blog/2021/03/advanced-network-error-sear...
https://dn.org/the-sitefinder-debacle-that-shook-the-interne...
I was thinking about interfering with existing sites.
Some carriers also passed info in these headers to be used to make billing calls to the Telco, such as subscribing to premium services with minimal (if any) consent from the user.
I got stung by this years ago. An ad for mobile ringtones showed up on some news site I visited on my phone, I scrolled past without clicking. Next minute I get a text telling me I've subscribed signed up to a (iirc) $25/month ringtone plan and that it'd show up on my phone bill. It took an hour on the phone to my carrier to block the charges and get a refund. The ringtone company kept messaging me for a year that I needed to contact them and "fix" the issue.
But the good news is that at least the majority (for now) is against it.
Even outside ideology, plain business sense makes ISPs not want to deal with censorship & co. It's work to do that, hence costs money. But there are also ISPs resisting out of ideology and civic duty.
its usually not nefarious, that is just the simpelest (and usually, not always) wrong explanation.
people who think this is not normal probarbly didnt experience what kind of shit u could find on the pre google pre monitored internet. for sure shit u do not want your kids to find. if u dont understand, then likely u dont have kids and very weird fetishes...
Assuming, you had cues for the gender & intent of the author, which I don't, but otherwise it's even more horrible to think about.
There are of course outliers but most people I know in the field care as much about personal rights & freedom as anyone else. Maybe even a small bit more.