They have low voltage contractor licenses in Florida too but that is a somewhat ignored law. This is particularly true in things like landscape lights and irrigation where the "yard man" typically does it. I don't know of a voltage limit and it really applies to voice and data too. I always wondered why nobody went after the trunk slammers who contract for Comcast and Embarq.
There is no licensure requirement at all in the NEC. The licensing of trades is a state issue and they are all different. The 50 volt threshold is an NFPA limit for where and when PPE's must be considered.