Where in the NEC does it say this?The only thing the code would say is if the heaters 120 volt then they would have to be GFCI protected.
2002 NEC 110.3 Examination, Identification, Installation, and Use of Equipment.
(A)(8)Other factors that contribute to the practical safeguarding of persons using or likely to come in contact with the equipment
(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.