What would make an engine dress up kit illegal for highway use?

I was looking for a engine dress up kit for my 1965 Plymouth Barracuda with a 273 commando but the only one I found says it's illegal for highway use? What would make a dress up kit illegal? Here's the link: http://www.partstrain.com/store/details/Plymouth/Barracuda/Mr_Gasket/Engine_Dress_Up_Kit/1965/Base/8_Cyl_4-dot-5L/G129831.html