Our Seattle area buses now have a diagram on the front of the bus that tells people not to place the rack arm too far toward the front of the bike's front wheel.
To be honest, I'm not really sure. It's just (to me, anyway) that those racks were probably designed to behave a certain way in case of a catastrophic failure/accident/collision, either in use or not. By locking a bike to the rack, it might (couldn't it?) affect the parameters of the design should such an event happen.
Mind I'm no engineer, nor am I a lawyer, but the transit companies wouldn't have bothered installing those racks if they couldn't have somehow limited their liability.
I don't see any accident-mitigating design in either of the rack models used by my local transit agencies, they'd probably just crumple and make a bicycle/rack sandwich. It's not something I worry about as my commuter bikes aren't all that expensive.
My local transit agencies limit their liability by placing all risk of use on the cyclists. The drivers aren't even supposed to help you mount your bicycle on the rack, although some thumb their noses at this and help out anyway.