Trustbusting the Parties?

I’ve finally found a copy of the debate ” of Understanding” (Thanks, Bill!) that the Kerry and Bush campaigns have agreed to, which makes me very happy, since as we all know, primary source information makes me get all tingly inside.
I’ll be going through it in detail over the next few days, but reading the first few pages, one thought jumped out at me. This is an agreement between “Kerry-Edwards ’04, Inc.” and “Bush-Cheney ’04, Inc.” which includes restrictive provisions such as:
“The parties agree that they will not… appear at any other debate or adversarial forum with any other presidential or vice presidential candidate.” (Pages 1-2)
Now, if Kerry-Edwards ’04, Inc. and Bush-Cheney ’04, Inc. were, say, Coke and Pepsi, and they signed an agreement which was clearly designed to exclude competitors from the marketplace, they’d be hauled up on antitrust violations.
So my question is: does antitrust law apply to political campaigns? They are incorporated entities of some type (hence the “Inc.”), but obviously not traditional commerical entities. How about for the actual parties themselves? Is there an exemption in antitrust law for political organizations?
And if anti-trust law doesn’t apply to the campaigns and parties — why not? I’m not asking for a practical, real-world why-not (that’s obvious), but rather, from a legal perspective, what’s the rationale as to why it shouldn’t apply?