I just visited NASM site and they are clear on why the certification was developed. It states:
FNS was developed to address the growing need for professionals with the ability to consult clients on:
Proper Food Intake
Weight Management Strategies
Function and Recommendations of Macro and Micronutrients
At www.cdrnet.org site it states:
Dietetics practitioners are licensed by states to ensure that only qualified, trained professional provide nutrition services or advice to individuals requiring or seeking nutrition care or information. Only state — licensed dietetics professionals can provide nutrition counseling. Nonlicensed practitioners may be subject to prosecution for practicing without a license. States with certification laws limit the use of particular titles (eg, dietitian or nutritionist) to persons meeting predetermined requirements; however, persons not certified can still practice. Consumers in these states who are seeking nutrition therapy assistance need to be more cautious and aware of the qualifications of the provider they choose.
Examine the scope of practice of the FNS and determine whether you will be able to legally use your skills within your state.
I hope this helps.