The negative ads are rarely through the candidates campaign directly. They are by a PAC which is not, in legal terms, affiliated with the candidate. Corporations can contribute to the PAC instead of to the candidate's campaign. It seems like it is all a big loop hole, but we do have freedom of speech. Freedom of speech is itself a catch 22. If you want it for yourself, it is there for everyone else even if you don't agree with what they say.