To be fair, there is no "50%" support. Let's be realistic here...
If I had to disassociate myself with everything I believe is unjust and wrong in this world, I would use no services, buy no products and live in a cabin on a desolate volcanic island built with my own two hands.
Bottom line: guilt by association is typically unfair and even unreasonable in most circumstances.
ESA is an organization that is very heavily invested in protecting intellectual property rights -- that's what they do -- so obviously they support SOPA. That's the ESA's business though.
I don't think it makes sense for Nintendo and others to leave the ESA over a singular "disagreement" they have with the organization's overall ideology. ESA is involved with ESRB ratings, IP protection, charity, scholarships, events like E3 etc... Most companies like those things. I'm sure most of them would choose to remain an ESA member regardless of how they feel about SOPA unless they were truly invested in making a serious statement against SOPA.
The fact is though, most companies aren't willing to make a stance like that, regardless of their principles. Honestly, I don't blame them if they aren't willing. Don't get me wrong -- it is great when a company who shares your principles does make that momentous, symbolic stance -- but I certainly don't *expect* any of them to be activists.
Judge Nintendo, Sony and others as you will, but just because they are members of ESA (like most other game publishers, studios etc..) doesn't mean they support SOPA. It simply means they don't hate SOPA enough to justify letting go of all the other things about the ESA they have in common.