For the same reason that “send me your advertising” ticky-boxes on website sign-ups should not be ticked by default: because the “feature” is detrimental to many (if not most) users and you have to spot the control before you can disable it. Worse, in this case many ignorant users won’t make the connection between this misfeature and the fact that their laptop is suddenly burning through its battery in double-time—they’ll just assume Firefox is now broken.