A few things Kasz.
1) The comparison between political lobby groups and corporations is wrong. Political lobby groups are groups of individuals working towards a common political goal. Corporations are entities answerable to shareholders, they are not a group of individuals in the same way political groups such as the ACLU are.
2) There is the possibility of maintaining freedom of speech while limiting political campaigning. Many countries simply limit the amount of money allowed to be spent - while corporations can express a political opinion they cannot spend tens of millions of dollars trying to convert other people to share their opinion. While this does put a limit on freedom of speech the limit is not overly stringent and on the balance of things the minor loss of that freedom is worth the major gains in political freedom.
3) It's pretty obvious that quite a few politicians in the USA do answer to corporate lobby groups. For example it's how bills like SOPA, CISPA and PIPA keep popping up - the massive IP lobbies fund the politicians essentially in exchange for bills that protect their interests.
Hell even in NZ we're currently having scandals over campaign funding (of course on a much smaller scale - the corporate donation was only $15,000) and possibly going to end up to entirely tax payer funded political campaigning because of it.
