yes there is a rule that states that along with hiking on unmarked trails on the map, but that is what the paths are used for. 
some of the paths are also not as informal as seen here, there is a path clearly marked out on ground. https://www.google.com/maps/@41.9801051,-87.744882,3a,15y,104.53h,83.55t/data=!3m6!1e1!3m4!1ssmUmrxy9sB5vhKVQaSvoxA!2e0!7i16384!8i8192?authuser=0&entry=ttu
 