I'm currently reading "The Box" by Marc Levinson off of Bill Gates' summer reading list. There was a very similar patent situation with container crane couplings. One x-factor with the patent decision may have been standards committees. I'm assuming that the seatbelt coupling was standardized as much as possible to make repairs easier. Standards committees (from what I've gleamed from "The Box") avoid endorsing patented technology. Abiding by standards committees can effect the level of government aide the car companies are eligible for, among other things. So the car companies really may have had to chose between adopting a less effective, but standardized, design, or ask Volvo to release the patent.
I'm sure there's someone on HN who's more qualified to speak about this particular case, but there have certainly been times where patent disputes are that black and white.
Depends on the standards committee - for things like 3G telephones you can't legally implement the standard without licensing the patent pool, although license terms have to be 'fair, reasonable and nondiscriminatory' [1]. Qualcomm make more money from their patents on 3G technology than they do from producing and selling hardware. Likewise, there are patents on MP3 and h.264
I've heard people joke that the aim of being on a standards committee is to get as much of your protected IP into the standard as possible :)
I'm sure there's someone on HN who's more qualified to speak about this particular case, but there have certainly been times where patent disputes are that black and white.