Is it a law or ordinance that your dog be on a leash in that area?
Some people are either scared of dogs or they just don't like them enough to be friends with them. Maybe due to things like the thread showing the dog attacking a child and being chased off by Sylvester the cat or they've been bitten before because they are scared of dogs.
Still, it's funny how some people think they own where they happen to be recreating.
Some friends and I were playing Frisbee golf in a state park not far from here. The course runs through one of the picnic areas and we stopped to rest on a picnic table near other tables that a lady was prepping for a child's birthday party. She took exception to our resting there and came at us like an old hen, even though we assured her that we would move on shortly after one of our party completed a run to the restroom. She still insisted that we vacate immediately or risk an encounter with the rangers. The way she acted, you'd think we were crashing the party and stealing the cake and ice cream.
Just to be on the safe side, put your dog on a leash the next time you use the trail and the yuppie bikers won't have anything to complain about.
If they do, proceed to the parking area and deflate the tires on any car with a bike rack on it.
