ICOM's games were published by NEC if I remember right. They would be equivalent to what people call a "second party" nowadays along with Cinemaware. (An aside, I've never liked that definition for second party, it used to mean user-written software dammit!
At any rate, they were captive developers.
And yeah, I know about the whole Nintendo third party exclusivity thing as I mentioned in my original post. As Black Tiger said, I'm more curious why NEC never attracted some of the more obscure third parties like Sega did with the Genesis in the first couple of years before the SNES hit the scene. A tell-all book on how NEC handled the Turbo like the books that cover Nintendo would be very interesting, of course there's probably not a dozen people who would be interested in such a thing.
Didn't IGS survive into at least the late 90s? I thought they released a couple of arcade shooters in that time. Or was that a totally different company from the one that published on the Turbo?