The Federal--NHTSA rule mandates the monitoring equipment. The ABS sensor based systems were not capable of detecting when any combination of tires, up to all four tires is significantly under-inflated.
http://www.interregs.com/spotlight.php?id=26
(additional item quoted from a 4/7/2005 CBS article:
"NHTSA originally issued a rule in December 2001 that would have required vehicles made after November 2003 to have dashboard lights warning drivers if their tire pressure was low.
But Public Citizen and other consumer groups sued the government agency, arguing the rule weakly allowed automakers to choose between cheaper "indirect" monitors, which operate off the antilock braking system, or "direct" systems, which have monitors attached to each wheel.
A federal appeals court in New York agreed with the consumer groups and tossed out NHTSA's rule in August 2003, leading to the new process of issuing the regulation.")