When you first signed a lease you probably were fooled by a friendly leasing agent, who told you that the "rent" listed on the lease is just a formality.
When you tried to negotiate your rent the following year another friendly leasing agent tells you that "concessions" scheme is perfectly legal.
It is easy to be fooled because the agents may seem like decent and honest people. The fact is that they likely don’t know that they’re not telling you the truth. This makes them perfect salespeople for the scam.
On the other hand, the building manager likely knows all about the "concessions" scheme. She may 1) trust that the corporation's lawyers are right 2) suspect that their lawyers are wrong but think that they are clever enough to dodge the law 3) not care because her bonus depends on it.
Don't get fooled. You're not negotiating with a person; you are negotiating with a corporation. Its executives and lawyers know exactly what they're doing. Be firm in your knowledge that the law is on your side.