Customers are customers. Insurance and Safety. The insurance issue is often thrown out as a red herring. Perhaps you're the first bicyclist who's ever used the drive-through facility. A bit of John Forester here should clear that up.
Bicyclists fare best when they act and are treated as drivers of vehicles.
If anything, bicyclists are more easily accessing the pneumatic tubes and other devices to the system. They can react faster if they need to. Try not to get mad, hard as that is. You've been denied service because of the vehicle you choose. Not a nice thought.