I've wondered this myself - what happens when ad blocking technology becomes integrated into mobile devices, or the general public starts to use it because the newest, shiniest browser now bundles it by default?
The technology is available (Adblock for Firefox Mobile) but given that Google controls the default browser and ad revenue is important to Mozilla, it won't be defaulted for a while.
Apple is another matter. Maybe they like to keep their options open, too.
Apple is another matter. Maybe they like to keep their options open, too.