This is the press-release for their formation, posted last month: http://partnershipforamericaninnovation.org/announcement ("To date, the conversation around patents has been dominated by those seeking to curtail America’s strong system for narrow, short-term gains. Companies like those in the PAI support a strong, balanced system and are working together to make sure the conversation is driven by facts, not rhetoric, and reason rather than emotion.")
This is PAI's most recent blog post, from 10 days ago: http://partnershipforamericaninnovation.org/belief-patent-sy.... It's titled, "The belief that our patent system is broken is patently false." It leads with: ("If you listen to some commentators on the subject of patents, you could form the belief that the patent system is irreparably broken. According to these arguments, 'patent trolls' are bringing businesses to a complete halt and that software patents are a barrier to innovation. Some pundits even equate 'patent troll' litigation with the legitimate legal interests of inventors who are defending their intellectual property.")[1]
To me, these read as a pointed rejections of the arguments used by proponents of the Senate bill, timed to have maximum impact on the members of Congress working on the bill. You're right that these companies don't suffer from troll suits and so don't have an investment in getting fully behind reform. But these messages, timed the way they were, did a lot more than simply declare an intention to sit out the debate.
[1] Note that statements like these do not get posted on a joint-blog without extensive scrutiny and sign-off by all the companies involved.
