It's not at all different. Wal-Mart benefits from freedom of the press just as much as the New York Times does, although they have tended not to avail themselves of it simply because it's outside the scope of their business. But just as one doesn't have to literally own a printing press in order to claim freedom of the press, one needn't be a card carrying member of "The Press" to say whatever the fuck one wants about this or that douchenozzle politician. Wal-Mart doesn't suddenly gain that right if and only if they start publishing the Wal-Mart Times. They have it right now.
And I don't think they should. I think it is an invitation for corruption and the loss of liberty to allow corporations to fund politics. I also think there needs to be strict (and not large) limits on spending by candidates and supporters of candidates.