Why is it okay to lie in political campaign ads?

I understand that it would be spectacularly dumb for Obama to start any kind of legal action against McCain’s campaign for any of the made up shit in his various propaganda, but even so, one would think that a system would exist to stop people from lying in TV ads. I’m sure such a system already exists, but why then is it never used? If I see a lie in a TV ad I can email the ASA and they’ll investigate and take action. I can only assume that an equivalent body exists in the US, and yet I’ve never heard a single report that started ‘the McCain campaign’s latest ad has been pulled after an adjunction following a complaint from a citizen’.

Seems strange to me, considering what is at stake.