In the meantime, riding on the firetower side of Ninham is legal, as none of those trails cross into DEP (NYC Watershed) land.
As for the private property that Deer Ear crosses, the explanation I heard was that some of the property was sold, and the new owners were not happy about the bike trails crossing their land. Hence the fence. Respect it, or we could have problems with the DEC (NY State) land that's still legal to ride.
There will not be fences blocking off the DEP land from the DEC land, as hiking is legal on the DEP land with a permit. So heed the no trespassing signs. Failure to do so could result in a ticket, and apparently they have the legal right to impound your bike until your court date, so if they do stop you, it pays to be nice.
Unfortunately, the issue with trails on DEP land is not one that is going to be quickly resolved, as the DEP's policy on all the land it manages is to exclude biking. They've ticketed sporadically for the last few years, but apparently have grown more active about enforcement thanks to the increased presence of ATV's. Some of us are working on ways to change that DEP policy, but it will likely take a couple years of lobbying...