It's next to the battery. It's like Cessna messes with you by sticking just one fuse out there exposed to weather and the rest inside the cockpit.
It keeps renters from killing the Hobbs after taxiing away from the FBO.
Really.
I mean it. This was a BIG problem for a lot of FBOs when they started using Hobbs meters (post-WWII) to figure both cost and maintenance on rentals. It's not that a lot of people were actually pulling the fuses, but if the Hobbs was being used "for trade" (that is, as the meter to determine what a customer would pay), the numbers had to be "legally defensible." That meant the Hobbs had to be completely out of the control of the customer when in use. Thus the fuse on the firewall.