Well, the whole science behind it is a little bit vague, there is a book from Lance Armstrong (you know, American who rides a bike really well - but since he is not in the NFL, NHL, or MBA, he is unknown) where he discusses the fact that a cardiovascular workout does well at burning fat, too.
I think one major reason why they say 130-150 is a good fat burning regime is so that they don't get lawsuits from 300 pound people who haven't worked out in a long time, and go "all the way" - to a heart attack.
If you want both cv and fb workouts, do the following. If you are going to be on the treadmill for like 40 minutes, start five minutes in the 130-150 regime, then do 20 or 25 minutes in the 150-165(170, depending on your age), and then go back for 10-15 minutes at 130-150 again.