We had a heated debate with one of those manufacturers on this subject in this forum (I think) many months back. Seems that because it is low voltage manufacturers think that no codes apply and there also is no safety issue.
Although the NEC sections are minimal, 300.5 gives the minimum cover requirement for buried landscape lighting cable and 411.1 gives the equirements for lighting systems 30 volts or less, these are code requirements.
To answer your question, I think it would be up to the AHJ's in each area as to how they want to deal with this issue. I think I vaguely remember some area requiring landscapers to take a short course and pass a test on these areas which would then allow them to install the lighting.
-Hal
