Is it not possible that Coinbase just shut you down rather than bothering to get a bit more info? I really can't remember anyone noting demands for extra information like Bitstamp and others. You might trigger something and then it's goodnight no matter what.
It pretty much depends on the severity of the term you violated. My point basically is that an exchange such as Coinbase will try to get to the bottom of your income sources, based on suspicious activity or completely randomly.
Another thing that could play a role is the country you are from, because there are various brackets of risk that each country fits in. I'm sure there are plenty more variables that come into play that they don't want to disclose.
Overall, it's not the exchange's choice to be this strict, but their banking parter that demand them to follow their rules, otherwise they can kiss their precious bank accounts goodbye. They rather kick you out than lose their bank accounts.