Much is made of Tubliss not being "DOT approved". Helmets are, tires are,
window glass is, headlights, turn signals......so why not tubes?

The simple answer is tubes are  mentioned but not regulated 
in the Federal Motor Vehicle Safety Standard, found at   
https://www.ecfr.gov

Most of the regulation on tires is related to labeling and the
precise meaning of terms in labels. Size and capacity must be
spelled out, and methods for measuring both are detailed. When
a manufacturer puts a DOT emblem on a tire he's merely claiming
that the performance claimed will be confirmed under DOT test
protocols.  

It's worth noting that in the context of a tire, the term used
is "innertube" (one word). "Inner tube" (two words) seems to refer
to impermeable linings of fluid hoses. Tubes are somewhat like lug
nuts; they have to keep things together, how is unspecified.


What does matter is the "Off road use only" notation molded into
Tubliss liners. That means Nuetech does not think the Tubliss 
system appropriate for use on public roads. If you make somebody
mad at you and you're using Tubliss, don't expect Nuetech to help.