Overheard another landlord about rent discounts....

[Casandra M.]

I overheard another landlord in the  S.E. Wisconsin area stating that last year they started a rent promotion program where if tenants have a certain amount of volunteer hours logged with a list of pre-approved organizations in the city where the rental units are, that they got a 10% reduction in rent. 

Assuming these are non-profit orgs that are not restricted to religious organizations or targeting protected-classes, is this actually legal? I know that it's legal to give rent discounts to serviceman and those in the police and fire dept., but I never gave a second though to other types of discounts?