Hrm... this seems to be a fine line between real legal issue and just plain silliness. On one hand, the legal premise is there that it is discrimination. On the other hand, what is this world coming to when people sue over ladies' day/night?
The spirit of the promotion in this case is not discriminatory. I think it could easily be argued that it's a good promotion to increase the numbers of an under represented group in shooting.
I don't think anyone is complaining about ladies' night at their local watering hole or other social venue.