The problem I have isn't with the idea of organizations making donations, but rather when those donations create a "direct line" from the lobbyist group to the politician, bypassing the constituents.
Representatives are there to represent contituents because the USA is too big for direct democracy. If industry x really has such a profound impact on a community then it's in the self-interest of voters to reflect that at the voting booth by voting for the politician that will help said industry, and by extension, their community.
A private organization greasing the hand of a politician in order to ram a piece of legislation through, against the wishes of said politician's constituents is beyond contempt. From all the evidence I've seen, the only ones who like SOPA are the entertainment industry, and last time I checked they weren't elected to any governmental position.