The US law (via wikipedia):
"Definition
21 U.S.C. § 802(32)
(A) Except as provided in subparagraph (C), the term controlled substance analogue means a substance -
(i) the chemical structure of which is substantially similar to the chemical structure of a controlled substance in schedule I or II;
(ii) which has a stimulant, depressant, or hallucinogenic effect on the central nervous system that is substantially similar to or greater than the stimulant, depressant, or hallucinogenic effect on the central nervous system of a controlled substance in schedule I or II; or
(iii) with respect to a particular person, which such person represents or intends to have a stimulant, depressant, or hallucinogenic effect on the central nervous system that is substantially similar to or greater than the stimulant, depressant, or hallucinogenic effect on the central nervous system of a controlled substance in schedule I or II.
(B) The designation of gamma butyrolactone or any other chemical as a listed chemical pursuant to paragraph (34) or (35) does not preclude a finding pursuant to subparagraph (A) of this paragraph that the chemical is a controlled substance analogue.
(C) Such term does not include -
(i) a controlled substance;
(ii) any substance for which there is an approved new drug application;
(iii) with respect to a particular person any substance, if an exemption is in effect for investigational use, for that person, under section 355 of this title to the extent conduct with respect to such substance is pursuant to such exemption; or
(iv) any substance to the extent not intended for human consumption before such an exemption takes effect with respect to that substance."
[emphasis added]
The structural similarity of NMT to DMT is unarguable, so clause A (i) covers that. When the CSA was drafted, there was (according to Shulgin) an 'and' after clause A (i) which has been omitted in the final document passed into law. This intentional omission makes life easier for the prosecutors so they don't have to go into just how much of "a stimulant, depressant, or hallucinogenic effect on the central nervous system that is substantially similar to or greater than the stimulant, depressant, or hallucinogenic effect on the central nervous system of a controlled substance in schedule I or II" any given substance might have, and also gives them leeway to pursue cases where there is no structural similarity, merely similarity of effect.
Then there's clause A (iii) which basically means that if you misrepresented caffeine powder as a new substance having similar effects to cocaine then they could get you as well.
Of course, the phrase "substantially similar" is intentionally vague, too. Is serotonin "substantially similar" to bufotenine? And just how "substantially similar" is the "stimulant, depressant, or hallucinogenic effect on the central nervous system" of NMT to that of any other "controlled substance in schedule I or II"?