It is a different safety issue. Most US airlines rules say that active consumer electronics (such equipment generally being self certified by the manufacturer for compliance with various FCC regs) must be switched off below FL10. The NCH products are *active* electronics and they are covered by those rules. Delta's manuals specifically state that NCH are not allowed below FL10 and it seems that the FAs are increasingly aware of that fact.

Is it unlikely that NCHs could disrupt avionics or radio communication? Yes. Is it it impossible? No - stranger things have happened! If you allow untested NCH products below FL10 why not a host of other untested products too?