That's true, but the USA makes its own immigration laws for the USA (obviously), and if the USA decides that travellers must leave the USA, Canada, Mexico, and the adjacent islands within the 90-day admission period of the visa waiver program, well that is the USA's prerogative.

As noted earlier, this has caused problems for visa-waiver nationals travelling via the USA to a contiguous country (say, Canada) to spend more than 90 days there: they cannot then leave the contiguous country via the USA again with a US visa. (Ironically, if they made a "side-trip" from the contiguous country during their stay there (e.g. spent a week in London), then they could.)

And in most cases it's not possible simply to transit through a US airport; one generally has to be admitted to the USA, even if the stay is going to be only a matter of an hour or two. (I can't see the point of this arrangement, since it seems to me to have the opposite effect to the one that is presumably intended: after all, once the passenger has been admitted to the USA, then he or she can, in practice, just leave the airport and go anywhere and do anything within the country.)