They are not legally liable, which is different from being responsible. Imagine if one of their portfolio companies was a child porn distributor. You wouldn't defend KKR, because clearly they have some choice in who they put in their portfolio, and a child porn distributor is an immoral choice. Holding a SOPA supporting company is also a (albeit less egregious) poor choice. Taking away invites of their portfolio companies provides motivation for them to look into this sort of thing when choosing companies in the future.