IMO, because that way they can claim to landlords they get target ocupancy rates with the higher price; if one landlord in the area knows the price others get, he could try to get higher occupancy rate by lowering his price, aka "competition", by requiring "compliance" to RealPage prices, they are effectively coordinating and price fixing the market, which is the ilegal part. After all, landlords could see regular posing on other channels and adjust their prices in the "normal" way.