Tbf the manufacturing standards for plenty of e-scooters and -bikes can be pretty iffy sometimes, and people abuse them in ways that can increase the likelihood of issues. I concede that the vast majority of electric personal transportation devices that go up in flames usually happens during charging. A public transportation bus has to meet higher standards than a mono wheel scooter off of AliExpress.
(Imo they should be allowed on, but I can see the point in not doing so)
I'm not gonna say bike batteries explode all the time but I will say I worked at an ebike company and they had an entire department dedicated to handling exploding battery lawsuits.
Honestly, they should only allow devices with removable batteries, and they could have a bucket of sand outside the bus that holds those batteries. Kind of like how bicycles are attached to the front, you'd drop the battery in and then board the bus.
That's what the external bucket of sand is for. It'll smoke and the driver will notice in time to safely get everyone off the bus. They could then likely just detach the bucket and reboard if everything looks fine.