That would be nice, but unfortunately it's not really the case. It's quite difficult to make a secure distributed system that allows anyone to join (as opposed to a closed darknet).

Here are a couple examples of attacks that DHT is vulnerable to:

http://www.cs.rice.edu/Conferences/IPTPS02/101.pdf http://www.cs.rice.edu/~twngan/doc/infocom06.pdf