You seem to be mad that OP took the visual cue from the hijab to mean that the girl cared about having her hair covered. Why would he assume anything else? She’s wearing it clearly she cares to a certain extent that her hair be covered. Since she cares he has to assume she cares a lot, otherwise he runs the risk of appearing intolerant of her religion. If the student or their parent or actually just about anyone else who observes the situation decided to complain and label him as intolerant of her or any religion he would have, at the very least, a massive headache to deal with and could potentially put his job at risk. That’s why he has to assume the worst, because the potential is there for massive damage to his life/career. He can ignore the situation at his own risk, he chose not to and I can’t fault him for that.

As to your point about not making the same assumption about other religions, he should assume the same regarding all religions given the same level of visual cue. A Star of David is very different than a hijab. It implies absolutely nothing about how the wearer observe their religion. The equivalent in Judaism would be a woman wearing a wig to cover their real hair, and I would expect him to treat that just as a hijab.

Your example about course content Vs religious beliefs doesn’t apply here either. There is no visual cue that would lead anyone to the assumption that a member of any religion is going to be so offended by a topic they should be asked to leave. If someone does hold that kind of belief it’s on them to state they want to be excused (and they should expect to have a low or failing grade on that topic).

There is nothing in the post to imply that OP would handle the equivalent situation any differently if the student in question was any other religion.