This is what I found online about the law stated in the letter. I am only quoting the sections that they list in the letter.
NATURAL RESOURCES AND ENVIRONMENTAL PROTECTION ACT (EXCERPT)
Act 451 of 1994
324.81133 Operation of ORV; prohibited acts.
A person shall not operate an ORV:
(a) At a rate of speed greater than is reasonable and proper, or in a careless manner having due regard for conditions then existing.
(e) In a state game area or state park or recreation area, except on roads, trails, or areas designated for this purpose; on state owned lands under the control of the department other than game areas, state parks, or recreational areas where the operation would be in violation of rules promulgated by the department; in a forest nursery or planting area; on public lands posted or reasonably identifiable as an area of forest reproduction, and when growing stock may be damaged; in a dedicated natural area of the department; or in any area in such a manner as to create an erosive condition, or to injure, damage, or destroy trees or growing crops. However, the department may permit an owner and guests of the owner to use an ORV within the boundaries of a state forest in order to access the owner's property.
(o) In or upon the waters of any stream, river, bog, wetland, swamp, marsh, or quagmire except over a bridge, culvert, or similar structure.
(q) In a manner so as to leave behind litter or other debris.
(r) In a manner contrary to operating regulations on public lands.