Apparently the maintainer trusted the first-time contributor enough to propose tackling another bug.

There is no trust needed when asking someone to fix a bug. It's not like the maintainer would lose anything if the contributor failed to fix the bug.

Besides, I think it is natural to want recognition when you do a lot of work for free. Many other people wouldn't do this unpaid work at all; recognizing their contribution is the bare minimum of good manners. Even in a company where employees are paid for their work, it is customary to give credit to co-workers who have helped you. Most people don't like to work in places where they don't feel appreciated, and that is also true in Open-Source.