It may or may not be an insurance exclusion. I doubt the risk to bikes is significant, and is likely far less than the risk during the ride on open roads to get to the drive-up.
Perhaps they should prohibit selling coffee to bikers... in case they spill hot coffee on themselves.
Personally I could care less as long as there is an alternative, although I can relate to the moments when I've either forgotten my lock, or chosen to ride without the lock, and wish to make a quick stop. I have actually skipped many stops because of not wanting to leave my bike unattended outside, so one would think stores would actively try to make more customers feel welcome.
There are, of course, places with either no bike racks, or poorly situated bike racks. I've even encountered public institutions without bike racks.
My peeve is when the establishments choose not to sell to customers without a car, and there is no alternative of walking inside.