The reason for the policy is to prevent security problems that could result from adults who look like our animated characters wandering about inside the park. During normal hours it is strictly enforced, however since people dress up for halloween, it is a little grayer during the MNSSHP event.
I am not an official company spokesperson, nor am I in charge of making decisions involving guest costumes. I don't want to give anyone the impression that their costume is A-OK if it won't be, so use common sense and discretion. My intent with the post was to let you all know so that you don't set yourself up for difficulties during your visit.