Honestly coming from a place where even normal banana bars are being removed from path entrances, I think “they want to keep out motorbikes” is an incredibly lazy reason to have such a terrible and inaccessible design. Police enforcement is the appropriate way to deal with rare cases where someone takes their motorbike onto a clearly illegal path.
As a general guiding principle, even if we ignore the accessibility issues for wheelchairs, fat pedestrians, or more unusual types of bicycles, if your intended design involves expecting cyclists to get off their bike, that is a horrible design. Bike infrastructure should never expect a cyclist to dismount any more than car infrastructure should expect a driver to get out and push.