It appears that the lawsuit is based on an equal protection claim (from the article I read). You may argue that because it is federal land, equal protection rights apply.
However, in order to defeat a equal protection claim for a non-protected class (snowboarders--arguably you are really just excluding equipment and not people anyway), the plaintiff would have to show that the exclusion does not meet a rational basis level of scrutiny or review. Rational basis review simply means that the enactment in question is "rationally related" to a "legitimate" governmental reason. No way this lawsuit exceeds unless they can successfully argue that the real reason for the rule is to exclude a protected class (e.g., 90% of black people that want to use the mountain snowboard and the rule is in place to exclude them).