I think the text of the original act is very vague as there wasn't a real definition of price discrimination in the act. However I wonder what the advantage would be to charge different prices for different tenants if their apartment unit is the same size and has the same built in features.
I think there are a number of good reasons. Even in theory "identical" units are located different places in the building with different times to reach the unit and different views of the city (which has to have some impact on value). It also may be the case that your bad luck means you are next to an annoying neighbor and the only reason you'd be willing to stay is if you paid a lower rate.
Additionally, and more importantly, there's a cost for changing each tenant. It makes quite a bit of sense to offer a premium for people who lock in longer leases to avoid these costs.
Also, wouldn't this prevent would be tenants from negotiating the price of their lease (or would it create a scenario where if a landlord wants to lower prices to get the one marginal customer he has to lower prices for everyone?).
Once you add these exceptions, a ban on price discrimination is pretty toothless, especially if landlords can marginally tweak amenities (or in future constructions make some rooms like 1 sq ft more or less) to make units "different"