They're illegal if they request money or other items of value and promise a substantial return to the participants. Those of you who know math will recognize these as attempts to create exponential growth. If not stopped, that will quickly overwhelm any network or mail system. Thus it doesn't matter to us whether items of value are involved. The USPS and FBI say that this is false.
People often think that nuking is a harmless prank.
Unfortunately the software used to do this often operates by overloading the network on the other end. Rutgers has a very fast network. We can easily generate enough network traffic to take another institution or company off the Internet. This has happened several times. We expect our users to follow community standards in use of netnews.