I am not a lawyer, nor am I a publisher. However, I do know how to do legal research and it appears that Mr. Hatch is correct in his suspicion that the law as it applies to advertisements has been misstated on this thread.
Heres a brief excerpt from 18 A.L.R.3d 1286, entitled RIGHT OF PUBLISHER OF NEWSPAPER OR MAGAZINE, IN ABSENCE OF CONTRACTUAL OBLIGATION, TO REFUSE PUBLICATION OF ADVERTISEMENT:
With the exception of one case, it has universally been held that in the absence of circumstances amounting to an illegal monopoly or conspiracy, the publisher of a newspaper or magazine is not required by law to accept and publish an advertisement, even where the advertisement is a proper one, and the regular fee for publication has been paid or tendered. The rule is not affected by the fact that the publisher enjoys a "virtual monopoly" because he operates the only newspaper or newspapers in the area. AND IT IS IMMATERIAL WHETHER HIS REFUSAL IS BASED UPON REASON, OR IS THE RESULT OF MERE CAPRICE, PREJUDICE, OR MALICE. (emphasis added)
The one case cited in the above paragraph was Uhlman v. Sherman
(apparently one of my relatives got in some legal trouble in Ohio, circa 1919). However, other courts within the same Ohio district have subsequently failed to follow that decision, so it is unlikely that it is good law. Regardless, the vast majority of American case-law would provide unwavering support for the right of refusal.
The only major limitation on the right to refuse advertising that crops up from time to time is the Sherman Antitrust Act (another coincidence?). Here, however, that law is inapplicable. It would only be called into play if someone said, for example, We will only run your ad if you agree not to run ads in our competitors magazine. That situation tends to encourage the formation of monopolies, but would not apply to a refusal in this case, if for no other reason than that (as I understand it) MUM, Linking Ring, and MAGIC also refuse to advertise for Magic Makers.
In sum, unless my reading of the law is completely inaccurate (which it could be, but I dont believe it is), refusal to carry Magic Makers (or any other unethical companies) advertisements would have no legal consequences.