In talking with other hikers, I found that they feel the same, but most don't like conflict, and don't speak up. They also don't feel that there is anything that can be done about the problem. I am sorry that I didn't get to hike the hiker-only trails, but I am willing to bet that I would have found bike tracks there, too, because the same thing is happening in every park I have ever visited. The mountain bikers are obviously able-bodied. Most of them are young and healthy-looking, and many are extremely muscular.
Any trail is half uphill and half downhill.
And it is well known that on level ground, the bicycle uses less energy per mile than any other form of transportation, including walking. Obviously, there is no better way to enjoy nature, than to walk. I didn't see a single ranger, except in the Visitor Center. We cannot afford to subject every square inch of our scanty remaining wildlife habitat to these hooligans. I did not visit any other area of the park, but I suspect that they are the same or worse.