You nailed it. If one could give what they can when they can that is another story. What if you had a medical problem, your car was totaled, you needed a new roof on the house, etc. but the church automatically took money from increased taxes?
Here you could just make up the difference when your fortunes improved.
They desperately need a First Amendment, I don’t care if some inbred English nobleman calls it “bonkers”.
Taxes may be the reason given for their leaving but there might be more. That percentage goes to the parish church they belong to they might not be agreeing with what their pastor is preaching