perhaps because the man carrying his car safely across junctions encourages others to push theirs and perhaps even drive it eeeever so slowly as long as they don't hurt anyone
they wouldn't have junctions and lights if everyone could be trusted to deal with the junction safely and with consideration
said it right at the beginning, someone dismounting safely, pushing it considerately and then remounting and carrying on is fine, but the vast majority of people I see doing this at lights don't, they don't dismount, or they don't push it carefully, most plain and simple ride up onto the pavement and rejoin the road on the other side
it's safe enough and not against the law so fine, what could be wrong with that?