From what I gather, the reason most of those companies are in the alliance is that the want Microsoft to continue supporting their hardware (especially hard drive makers), and less frequently software.
Apple is not a member, and neither are Sun nor SGI. European mobile phone makers Eriksson and Nokia are missing. So are Canon, Xerox and TI. Practically all the companies in it are major Windows/x86 platform players. If they don't make hardware, they assemble full systems, with a few playing in the software distributed computing scene only (Adobe to mention one). Mentioned non-members are the largest competitors to the Wintel sphere that do not themself take major part in it.
Sure, if the American congress should decide to make it law then I guess mentioned companies, Apple included, would be forced to. I have a hard time seeing that happen, it's an extreme movement even for the American political right.