They just advise it on the website, it might be them being over cautious, who knows?
When I did Lighter Life I know as I got close to goal I started fainting and my hair fell out, do maybe bodies are reluctant to use fat when there isn't that much of it left?
I guess if you listen to your body, and maybe switch to WS if you start getting ill, that might be a compromise. You could always check with them if you want to be sure.