Why can Crossfit give out nutritional advice and diets plans?

Giving out information regarding nutrition and prescribing a diet plan are two different things. You can inform your client why it is important to not cut out any macro-nutrient from their diet or why a client might not need as much protein as one think he/she needs. All that information is well documented and approved in some form that prevents you from being liable (like in my case, recommending and showing clients how to follow the Canada Food Guide).

In my specific case, I am recommending a piece of document that is approved by the government, so my liabilities are limited should any concern arises from a client following the Food Guide. The fact that the guide is written as a recommendation rather than a prescription makes this practice well within my scope.

The problem arises when one oversteps their boundaries and starts creating diet plans for their client. When you start prescribing food to your client, you aren't acting in the capacity of a personal trainer. For one to be able to prescribe a diet plan, one must have more understanding of how the body interacts with the food one eats, the understanding of how one's medical conditions may affect the way the food is absorbed or processed. One also needs to understand the energy demands of a client's daily activities to be able to prescribe food in the right amounts.

Unless your degree required you to apply your knowledge and work with people who needed a diet plan written for them, most of that information through your courses isn't put to much use. Even then, unless you get yourself certified/licensed to do so, you aren't recognized by any organization to actually prescribe meal plans. At the end of the day, your degree only states your level of education, it doesn't necessarily tell people what you know or can do. Your certification does, because your certification clearly defines your scope (assuming that your certification comes from an organization that is respectable).