About 10 years ago I got $100 for free to use on AdSense. I used it for fun not realizing it keeps going and then billed me. Since then I basically don't use any Google paid products. Hope that $250 was worth it.

That's... pretty much how every free trial works? Try signing up for a free month of Amazon Prime or Netflix and see what happens. The entire point of the promotion is retention.

AdSense doesn't present itself as a permanent service you stay subscribed to.

(Or at least didn't at the time I've tried to use it. That may have changed, but we don't know when the GP tried it either.)

It wasn't a subscription. My expectation was that it would simply stop once it hit $0. Not really here to argue about it. The tldr is I don't trust Google with this stuff anymore.

I wonder what you thought putting your credit card on file was for.

The credit card on file was for other stuff in Google.

Wait till you try aws!