because it's cheaper?
i've gotten away w/ it as well from stepping down to a t rating
basically the speed rating coincides w/ your speed limiter of the vehicle
i don't go ~140 mph so i don't need the H rated tire and went down from a soft tire to a harder T-rated touring tire
the softer tire i had were dunlops and they wore out on me after ~30,ooo miles
i went to a touring tire w/ an 80,ooo mile tread warranty & they did lasted me 7 years before showing signs of sun dryrotting
here's some good reading for you:
http://www.tirerack.com/tires/tirete...jsp?techid=35&