Prior to the Apple announcement, Pacific Gas & Electric, Exelon, and PNM Resources also said they would resign from the Chamber for the same reasons. Nike said that it would step down from the board of directors although it planned to advocate for climate change legislation within the Chamber.
For both the companies involved and the Chamber, the issue at risk is brand.
In withdrawing from the Chamber, Apple is shining the brightest light yet on the growing schism in corporate America over climate policy. Apple is the first highly-visible consumer brand to make the move, so will gather more attention than the other companies that have recently split with the Chamber.
... the most serious damage to the chamber is not on its balance sheet, but to its reputation. To argue that limiting carbon would be bad for business becomes much tougher when those you claim to represent publicly disagree, and distribute press releases about it."