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OBD II - What's going on here?

Someone please enlighten me on permissible modifications on OBD II
equipped cars.  The aftermarket is starting to come out with OBD II
chips for some cars (like VWs), and this Digest has reported that 
Superchargers are in the works for OBD II BMWs.  I just don't understand
how this can be...as I understand it, the engine management software needs
to have "checking routines" that monitors for anything "unusual" in engine
operation (e.g., O2 content upstream and downstream of the Cats, if
out-of-bounds with some factory-defined and *Fed approved* tolerance, will
trigger a fault code).

So it would seem that doing any significant modification (certainly
a Supercharger) is going to trigger a fault.  Unless, of course, the
aftermarket is fooling around with the "checking routines" (maybe even
by-passing them completely)....but, this *can't* possibly be legal,
can it?  How can the aftermarket get away with this?  Can someone
explain?
TIA,
- -Roger W. Graves
'95 M3
'91 Miata
RogRacer@domain.elided
rgraves@domain.elided