If, as the EPA recently declared, greenhouse gasses may be regulated under the Clean Air Act, and if methane is a greenhouse gas, then may the EPA regulate our diets?  May it tell us not to eat beans and sauerkraut?

I am speaking, partly, tongue in cheek, but I don't see where the flaw in the logic is.  Once the principle is established, by statutary construction and not by statute, that the EPA may regulate greenhouse gasses, the right to regulate all activities that produce greenhouse gasses necessarily follows.  As Churchill said, we're just haggling over price.

In the recent "Citizens United" case, the Supreme Court threw out a good deal of campaign finance regulation because, in partt, it was impossible to decide who is, and who is not a media organization. Everyone agrees that it would be bad to regulate the ability of news organizations to comment on ongoing political campaigns.  The trouble is, that exemption was not, in the language of statute,a matter of right. It was an exemption written into the statute. Once again, it was a matter of haggling over price.  Once it is established that the government may tell companies that they may not comment on campaigns, there's no way, in practice, to exempt media organizations, other than the government's arbitrary say so.  Hence that part of the statute fell.

Ultimately, I am reminded of Hayek's comment in The Road to Serfdom suggesting that "the democratic statesman who sets out to plan economic life will soon be confronted with the alternative of either assuming dictatorial powers or abandoning his plans."