If the range were turning away people based on gender, race, or some other factor, that would be one thing. They're not. They're simply offering a small monetary incentive to one group in order to attract business.
Let me flip it around on you again.
If they offered "White Guy Wednesday" would that be ok? Every wednesday the white guys get free range time... you know because they don't have enough white guys going to their range. How do you not think that wouldn't be brought to court as discrimination? But hey, we weren't turning the black guys away, we just required them to pay!
Any time you give one class of people, whether by race or gender, special privileges or pricing, it's discrimination. IE wrong. You know, 60 years ago black people weren't refused the right to ride the bus in the south, they just had to sit in the back, so no big deal right? (and yes I went there. It's fairly extreme example but it gets the point across. You can call it anything you like but it is what it is)