They class it as an "account management fee" (what, they have someone managing my account with $30 in it?! WTF!), but in reality it's because they know that most webmasters will earn next to nothing and eventually give up on clickbank and move onto something else as the OP did. Whatever the webmaster might earn in future - they'll never know because they forget to check their account.
That would mean that any commissions they generated would end up just sat there dormant in a bank account, so instead Clickbank want to take all of those earnings for themselves, and so they do it by syphoning off the money in fees.