In this case he wasn't arguing that people should give more weight to his answer because he went to law school. Rather, he was responding to someone that asked if he knew anything about the law. As a response to that specific question, giving a context for how much he understands based on his background seems valid.
You never know when the person you're bitching at on HN has a law degree. But now that it's clear that you don't: your objection is silly and the person you are objecting to knows more about this issue than you do.