Because she is simply an “exception.” Use of the phrase would require there to be a general understanding that “billionaires are bad except when…”, but the prevailing notion is simply that “billionaires are bad.” Therefore, she is an “exception”, and a good one at that, not an “exception that proves the rule”.
edit Oops, replied to the wrong one, but leaving this here because I’m lazy. I am the rule.
Always open your mind to other’s situations. You probably don’t have 4 roommates, but they might. 😉