Here are my thoughts on the situation. As usual, please correct me if I'm wrong on points.
Aliens: Colonial Marines really is a remnant of SEGA West's past - during the rocky SEGA of America/SEGA Europe days. I mean, look at early articles on the game and look at SEGA West's output at the time. Way the hell back in 2006 SEGA and Gearbox announced that a game (which ended up being this game) was in development. What else did SEGA release in 2006? Yeah, Sonic '06.
Not SEGA West per se, but it wasn't a strong year for SEGA titles in general. Really, Creative Assembly were the real Western stars at this time (still are, IMO. They do their thing and do it well).
Then development continued, and meanwhile SEGA of America/SEGA Europe (mainly SoA) continued to release shit like the Marvel titles, Daisy Fuentes Pilates, Golden Compass...
Eventually, SEGA wised up, reorganized into SEGA West, cut the crap and in my opinion were far smarter with their releases from 2012 onwards. But Aliens: Colonial Marines, that game born in a period when SEGA was making some pretty awful titles, was still alive and kicking and SEGA just wanted to release the damn thing and be over with it. Not to say that SEGA is to blame. Gearbox had more than enough dev time. SEGA just wanted a return on their investment from '06.
I'll give SEGA West credit, they did a pretty good job marketing the title as best they could. The community team's efforts have been strong on the social networks over the past months.
Really, I blame Gearbox. SEGA's purchase of the Aliens franchise electronic rights may not have paid off in the long run, but Aliens vs. Predator and Aliens Infestation were decent games that were a step up from a lot of the other externally developed content from the same era of SEGA. From SEGA's perspective, they had the rights, they had a competent developer (or so we thought) and they had the potential to make a game that could have been a step up from the past two franchise titles. Instead it appears Gearbox didn't give a shit and in turn screwed SEGA and more importantly screwed themselves by making a game that will hurt their reputation.
I hope SEGA does no more business with Gearbox and focuses on their internal Western teams like Creative Assembly, Relic and Sports Interactive. Those are innovative, competent developers. Gearbox... not so much, it turns out.