There's no advantage to them in people using iMessage. They can easily get your text messages already.
You could go high-tinfoil and claim they would rather people iMessaged than using whatsapp or some other free message service? But anyone thinking about avoiding federal intercepts would use neither of these things rendering this piece worthless.
So what do they actually have to gain?
It seems more likely to me that this is a real problem than a PR piece.