There is no difference between an online program like the one you’ve mentioned and giving a diet plan yourself. A nutritional advice is just that, an advice. I prefer to work with dietitians especially when the client needs something specific and personalized. Now there are apps that help with keeping track of the caloric counting and I believe they an be useful because they keep the client in some kind of accountability when it comes to food. But even with these apps a dietitian should be the one to design a diet plan and the amount and type of calories this person needs each day to be on the safe side. As far as being legal or not, there are no such regulations (from my understanding) at any state that prohibits trainers or anyone else to give a nutritional advice to another person….otherwise all of these apps would be illegal. This industry is very unregulated.