Originally Posted by mac
#1 - it's not your property. Respect the wishes of the owner or get off of his property
#2 - liability. We live in the lawsuit capital of the US. If you get hurt on his property doing that dangerous bicycle-riding thing, you can sue him for all he's got because he didn't warn you to not ride. That sign protects him for those lawsuits. Sadly, that's the world we live in.
#3 - Difficulty of banning kids from using bikes as toys on the property without banning bikes altogether.