[iy56]

That's an incredibly naive expectation. ISPs have been replacing error pages with their own search pages serving ads since the 90s.

[DanBC]

Ah, yes, you're right. Sorry.

For what it's worth I was grumpy in those situations too. I wrote polite letters. Where possible I opted out.

But your point - this kind of this happens all the time, and has been going on for years, and noone is doing anything to stop it even though it's wrong - is taken.