AWS and GC don't turn them off because they know they will eventually get paid. No reason to stop Twitter from continuing to rack up charges.

Years ago I wrote software for a big cell service provider, and (in my young naiveté haha) wondered why we didn't shut people off who didn't pay their bill (this was before pre-paid). The answer was the same, keep them racking up charges. If it got to a certain threshold, get them to pay a minimum, but do everything we could to never shut them off.

Bad PR, discontinued advertising from said companies and eventually being forced to pay anyway due to contractual obligations?

Conversely, I don't see any upside to doing this song and dance other than cash flow issues.

I don't know if you ever experimented with not paying an AWS bill. If you haven't: they will let you run your services and accrue new charges as if nothing happened. It's a well thought-out strategy.