As with model rocketry, safety codes have been written by both Tripoli and the NAR to help insure the continued safe enjoment of the hobby. While an F motor may get that 2lb model rocket off the ground, it won't do anything for one that weighs 5lbs. But larger rockets and motors, although as safe to store and handle as model rocket motors, require a higher degree of knowledge and maturity in their use.
Larger models require stronger materials and construction methods.
A model rocket with a thin cardboard tube and balsa fins glued together with yellow wood glue will fly just fine powered by a D motor, but would disintegrate under the power of a G motor or larger. High power models require thicker body tubes, model aircraft plywood or thick plastic fins, and stronger shock cord material just to name a few differences in materials. Larger models, if flown carelessly, could be dangerous. While it could be argued that many adults can be careless and stupid, it's also obvious that the level of maturity required by the hobby automatically restricts it to adults. Requiring membership and certification helps to insure that those enjoying the hobby will be knowledgeable and responsible.