Salt or chemicals might or might not remove all of the ice, depending on the depth, and can adverse effects upon the local ecosystem. Plowing will remove snow, but not ice in an environment with consistently below freezing temps. A sign warning of ice seems like it'd be more reliable; and if it's a recurring problem at that spot, then it'd be worth the time to contact your local Dept of Public Safety to point this out. Even up here they put "Ice in Tunnel" signs up along the MUP in the swing season, although they take them down as soon as winter proper arrives.

Still, if the temps are below freezing, it really should be up to the individual to watch out for ice. I don't see how the organization that maintains the trail that the cyclist fell on is any more responsible for icy conditions that will knock a cyclist down than they are for windy conditions that will knock a cyclist down. I've personally bit it trying to corner too fast on wet pavement in the middle of summer, and it was no one fault but my own. It's the outdoors; weather happens, ride accordingly.