My understanding is that the law was written in such a way that the pilots who volunteer are actually deputized as federal law enforcement officers after their training. As such, the fedgov is obligated to provide the tools required to perform their new additional duty, just like the air marshalls, FBI, IRS, and all of the other tens of thousands of federal agents from every agency you can imagine must use fedgov-issued guns.
Thanks. That makes sense. Then folks like Chicago's king richard can't hassle them.