That's news to me. If that's how they view it, it would be really nice if they would stick with the phrase voluntary evacuation, or recommended evacuation, or even strongly recommended. Mandatory takes away options.
Mandatory evacuations generally mean, don't expect help if you need it from emergency services (fire, police, rescue). I think this is reasonable as first responders already risk their lives in many circumstances under normal weather conditions for most of us. It is unfair for them to put their lives in further jeopardy under such circumstances. Unless you are arrested, I don't think there is any law on the book that can force you from your home.
i.e. I think it is basically is a fancy way of saying "you are on your own!"