To use the literal case of "snake oil"--
There is nothing illegal about selling snake oil, in and of itself (unless it were to be a dangerous product.)
The legal issue is that a snake oil salesman implies that it "performs" some feat (implied warranty for a particular purpose). If the snake oil salesman truly believes that it works, but it can be proven that it does not, he has misrepresented the product. If the snake oil salesman knows it doesn't work, and claims that it does, then he is committing fraud.
If a person choose to use this product and did not gain the advertised result, then most people would at least agree they are entitled to a refund or, maybe even damages that resulted due to use.
In the case of Mr. Trudeau, it is alleged that his claims are false. Because customers are buying his book "for the purpose of implementing his 'treatments'" the book carries an implied warranty that the content of the text is fit for a particular purpose. If you bought the plans to assemble a boom-box from parts at Radio Shack, and were told that the plans worked, and you followed them properly and it did not produce a boom-box, you could claim that the product did not meet its implied warranty duties.
If customers were buying his book for entertainment, not any particular purpose, all the book would need to have is "words" in some narrative format.
If he said "I am selling an international anthology of alternative medical practices for historical, literary, or critical purposes" [and it's not my fault if you try them, and should they work, good for you] or "I am selling an international anthology of medical research that is the sole opinion of the individual authors" then it would be a different case.
However, someone would have to first make a successful claim that the treatment does not work, or that harm was done by not using an alternative treatment, or harm was directly done by the product (which addresses another issue of strict liability.) In this case, the government is making such a claim, right or wrong.
If he was not selling the book, but made it freely available, it would be a pure free-speech issue, which is a much more open to interpretation than fraud or misrepresentation in a transaction.