I think it’s that there are people who legit commute over the bridge, and they can’t be left stranded. No one commutes over the bridge by foot.

But I suspect you’re also right they don’t want pedestrians at night due to the suicide risk.

I once had my derailleur break at night while crossing the bridge and had to walk. They sent out a truck pretty quick to check on me.

Probably an attempt to keep homeless people from sleeping on the pathway and getting run over by the night cyclists.