It's not forbidden, it just has a ~10% tariff (fine). $10 per MWh compared to the current price of $120 per MWh. If solar or wind ends up actually cheaper than coal, it'll probably be by more than that 10% so it'll still be economical to use them.
Furthermore, the fine doesn't apply to exported electricity, which is most of it: "Wyoming sends two-thirds of the electricity it generates to nearby states" [1]