Answer:
Average earnings for American nutritionists as of 2010 are around $50,000 a year.
Dietitians reported average earnings of $70,000 per year.
I'm not positive on American policy, but I know that in Canada anyone can call themselves a nutritionist, it's not a protected title. However the title dietitian is protected--you have to earn a license to be called a dietitian. This typically means dietitians have more formal training than nutritionists, although that's not alway true.
It does however explain why dietitians earn more. Being licensed means you HAVE had that formal training, so people are more likely to trust you and companies are more likely to hire you.