In my experience, it is only motos that are not street legal that have to have the USFS stamp of approval. If it has a license plate on it it doesn't have to have the stamp.
Look at it this way, if everything had to have a USFS stamp then your Jeep Wrangler or any other road legal vehicle would have to have one, and they don't.
Of course it is always up to the interpretation of the USFS LEO that you may have to deal with.
Below is the link to the official USFS Spark Arrester Guide.
http://www.fs.fed.us/t-d/programs/fi...ets/2012_GP.pd