There is no technical reason for this decision by Corsair, it's purely a business decision. They even acknowledge that - for the most part - in their reply. They try to blunt it a bit by throwing out some technical complications, but that's nonsense, really. It's about wanting users to buy all Corsair for their RGB builds.
Could they license iCue out to others? Sure, but other businesses have the same motivation to keep you in their ecosystems and using their software, so all this is unlikely.
What I AM surprised about is that some enterprising individual or business hasn't come along and written their own application to allow all these to work together harmoniously and built a truly usable interface. I've seen some out there that work, but the UI leaves much to be desired for the average user.
For the record, I do not blame Corsair for this position at all. I don't like hearing it, but I can 100% understand their perspective and decision around this.