I find the "overnight" more shocking than terminating a contract. That they would be so callous toward a customer should make anyone think twice before signing up.
I find it highly unlikely that they didn’t have conversations beforehand. It’s also really black and white - they were proud of their lack of moderation and said it publicly. It’s not like there was a misunderstanding. Plus an org like Parler has its own lawyers. There is no way they weren’t warned of their contractual obligations by their own counsel. They just chose to take that risk.