As far as I know it is enforced by the cell phone companies. The blocks of numbers allocated to cell phones cannot be used in autodialers. To get a call from a business you have to give them your cell phone number.
I believe the determining factor is the competition in the cell phone market, not regulation although there might be some regulation also.
I'm not sure why cellular phones aren't targeted. It simply may be out of courtesy since most cell phones are per minute vs bulk rate for home or office phones.