> They are looking at what competitors are charging before setting their own prices. That should be illegal?

That practice isn't what's illegal. What people are having issues with is that RealPage which is used by a large number of landlords (from private landlords to megacorps), recommends prices to the landlords when it comes to marketing a unit. This data is pulled from other rentals on the market and what other RealPage users are charging. A majority of the time, landlords just use that number to advertise. That's why some apartment communities have prices that change daily, since RealPage changes their numbers daily. As such, this is a roundabout way of price fixing as RealPage is in a position that artificially raises rents across a wide location range.

This isn't a case of companies looking at what other companies charge for the same service. This is one company that's comparing their pricing against themselves. This results in cases such as a primarily affordable apartment community can see rents shoot up $300-$400/month all because a luxury apartment opened on the other side of town and both management companies just so happen to use RealPage.

It's removing relevant comparisons from the actual pricing schedule, which in some cases can be seen as price fixing as the prices no longer reflect the actual market.