I assume that (barring some of the really fly-by-night ones who focus on being gone by the time anyone goes to investigate; rather than just being technically legal) do sport some variation on the "Not Intended to Diagnose, Treat, Cure or Prevent Any Disease" quack-Miranda you see on every "dietary supplement".
It seems likely that the FDA is concerned(probably not entirely unreasonably) that the 'it's not a medical device; it's just for quantified wellness!" tech guys, mostly harmless when selling new numbers to obsess over to the basically healthy, are going to cause some real trouble if their technically-not-illegal marketing claims sway anyone whose life does, in fact, depend on accurate blood glucose numbers and responding to them appropriately.
It wouldn't be surprising if, as a cultural thing, cautious FDA validation wonks don't really much like the people selling lightweight lifestyle hypochondria at tech gadget prices; but they don't have regulatory authority and if it's just mostly-healthy people fretting over numbers that normally take care of themselves it's a pretty low priority. If actual diabetics get into it, though, (as you could see them wanting to; noninvasive is definitely a compelling sales pitch) any shortcomings are going to turn into bad outcomes fairly quickly.