Why does everyone automatically assume that it's always a biker versus non-biker issue?

This could be as simple as an Liabilty & Insurance issue (a policy that all companies should have in place). Perhaps the employer does not want to responsible for the bike in case of fire, theft or if someone borrows the bike without the owners permission & then gets into or causes an accident.

I could be wrong, but once the employer acknowledges that the personal property is in the building, they're responsible for it.