Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances. The First Amendment to the U.S. Constitution.
So if "Congress shall make no law ... abridging the freedom of speech", why is lying about doughnuts illegal? Why is shouting "FIRE!" in a crowded theater (assuming there is no fire) illegal? Clearly these are abridgements of what one can say.
On the other hand, why is political deception protected? Shouldn't that be illegal too? After all, doesn't political deception have the potential for far, far greater damage to society than someone lying about doughnuts?
I believe the answer to that is "Maybe, But" with the "But" hugely outweighing the "Maybe".
While political deception has great potential to damage society, if lies regarding commercial speech were protected and therefore common, whether it be about doughnuts, contractual agreements, etc., it would either stifle or destroy commerce since nobody could depend on anyone else to follow through on their obligations ("I said I'd do X, but I lied, too bad for you"). Our society is absolutely dependent on commerce. So if commercial deception was protected speech, it's hard for me to imagine that there would be much of a society left. The potential for political deception to be even worse is therefore limited.
There are checks and balances built into our system to mitigate the damage caused by political deception. Opposing candidates and parties are highly motivated to check every statement of a candidate or politician. A fine example of this is the many groups combing Bush's SOTU address and investigating every sentence and discovering that those "sixteen words" were potentially misleading. That level of resource and focus cannot be applied to every doughnut salesman or commercial venture.
There are also the checks and balances between the executive and legislative branches. The President can't make laws and thus has limited power. The President can attempt deception from the bully pulpit but it's Congress' job to see through that deception and they usually do. While the President is the commander-in-chief, Congress can take away military funding and you can't fight a war without money.
If political deception was to become illegal within government, there's a serious practical consideration. Who would enforce the laws? The executive branch normally enforces laws, but would it enforce the laws against deceptive statements by Senators and Representatives? Wouldn't that enable rampant intimidation and blackmail by the President against Congressmen ("Senator, I'll let that lie in your last campaign go if you vote for this piece of legislation")? Who would enforce deception laws against the President? A special prosecutor? If prosecuted and jailed, would he still be President? After all, he wasn't impeached for treason or other high crimes, just jailed for lying. Or would the President just be impeached and then prosecuted and jailed as an ordinary citizen.
The last consideration is also the scariest. Politicians, including the President, are also ordinary citizens in this democracy. If ordinary citizens could be jailed for deceptive political speech or lies, the effect on political debate would be devastating. The government could comb the speech of dissidents for potential lies or deception and jail all dissidents and stifle all dissent. As an example, Donald Luskin has nearly made a career by checking every one of Paul Krugman's columns, books, writings, speeches, etc., and pointing out every false statement (i.e., lie). By the standard that would put Bush in jail for lying, Krugman would be in jail many times over. But Krugman is a critically important voice in the political landscape right now (I don't always agree with Krugman but I do think he's critically important). Stifling voices like Krugman's (or even Luskin's for that matter) would stifle democracy.
So no, I don't think lying about nuclear fuel (political deception) and lying to sell more doughnuts (commercial deception) are even vaguely similar in terms of legal, political, and societal implications. All of our other freedoms are completely dependent on free political speech and debate and I firmly believe that any speech within in the political realm must be tolerated at all costs, whether it be deceptive, hateful, stupid, wrong, ignorant or otherwise negative. And I don't see that to be the case for fraudulent selling of doughnuts or other commercial speech.