It's in the NAR rules for sure. Section 2.6:
"A rocket used for a Level 3 certification must have a minimum fineness (length-to-mean diameter/body width) ratio of 4:1 and be aerodynamically stabilized using fins, tubes or other non-shroud components of measurable thickness not to exceed 10% of the mean chord or semi-span. In lieu of calculating the mean diameter/body width, maximum diameter/body width may be used. The documentation submitted for review shall include the fineness ratio and mean chord/semispan-to-thickness ratio "
So, "must ... be aerodynamically stabilized using fins, tubes or other non-shroud components ..." rules out sticking a motor in a nose cone.