Decibel levels aside, the reason the Hindle is not street legal is that it lacks the catalytic converter the stock exhaust has. This does not allow it to meet USEPA emissions standards for new vehicles. The manufacturer must label the exhaust in this manner in order to sell it, and technically the dealers are not allowed to install them, nor are the vehicles to be used on the street if so equipped.
As to prosecution, it is doubtful that there would be trouble in most states or communities. Laws regarding exhaust noise vary, but the Hindle is comparatively quiet, and can meet some, but not all, of the noise laws. If your state has emissions testing for motorcycles, you may be in a different bind, however. I seriously doubt a Hindle equipped Spyder would pass an emissions test.
-Scotty