How weird.
Put a line down the middle, clearly mark it for cyclists one side, pedestrians the other, and people will walk 8-abreast across the whole bridge and moan about the cyclist "menace" should anyone wish to ride past them.
Make it really awkward and they will tippy-toe along the edge as if the white line were electrified.
I can only deduce that people just prefer to feel inconvenienced in some way; either by the cyclist in the first instance, or the white line in the second. Just being sensible is too dull and boring.