Google does have a powerful distaste for affiliates. I'm not saying that's the reason for the probationary period but it might have something to do with your account being "shut down" whatever that meant.
I think it has to do with fraudulent accounts / accounts created with stolen credit cards. I read somewhere that people are selling/buying "precharged" accounts. Like an account w/ $500 in credit for $50. I am guessing these would all be done with a stolen/phished CC #. By limiting the spend to $50 and then verifying the data, it is probably saving google lots of time and money in charge backs, legal fees and such.
I had a client that was able to run a few days until he hit about $100 I think it was. Then he had to call both Google and Amex to verify he was the account holder. Now it runs fine.