You are correct in that mobile device use is only one of many distractions. Several of the behaviors you list are also illegal and occasionally ticketed. 

Statistics say enforcement reduces (not eliminates) some of the behaviors. That means it decreases (not eliminates) the risk to the rest of us, including people who commute on two wheels. Those reductions might not feel like enough but they're better than nothing. If it generates a little revenue courtesy of people who don't care about your life, all the better.