In terms of Polo, I don't know if they offer any discounts or rebates privately beyond what they give everyone, but it is not unusual, or even improper, to give incentive discounts or rebates to high volume market makers. I've been offered this in the past by other crypto exchanges (not Polo).
I am not using Polo lately since I have stopped trading for a while, but I do believe that they are offering some discounts to like you called them "high volume market makers". Of course that this is not something, which we can call improper... its just doing good business.
