It may be legal. Before Leegin Creative Leather Products, Inc. v. PSKS, Inc., 551 U.S. 877 (2007) it was illegal since 1911 under Dr. Miles Medical Co. v. John D. Park & Sons Co., 220 U. S. 373 which held that minimum pricing was per se illegal. Leegin overturned that ruling. The basic engine behind this decision is antitrust legislation (§1 of the Sherman Antitrust Act), and the premise that manufacturer price controls might reduce competition. Now the matter must be decided on the basis of judged based on the rule of reason, which is a doctrine saying how that act is to be interpreted. Now,
The accepted standard for testing whether a practice restrains trade in violation of §1 is the rule of reason, which requires the factfinder to weigh "all of the circumstances". This
rule distinguishes between restraints with anticompetitive effect that
are harmful to the consumer and those with procompetitive effect that
are in the consumer’s best interest. However, when a restraint is
deemed "unlawful per se," ibid., the need to study an individual
restraint’s reasonableness in light of real market forces is
eliminated. Resort to per se rules is confined to restraints "that
would always or almost always tend to restrict competition and
decrease output." Thus, a per se rule is appropriate only after courts
have had considerable experience with the type of restraint at issue
Bear in mind that states can also have separate antitrust laws.
A manufacturer could independently make price guidelines part of their agreement with retailers, and can stop dealing with sellers who do not follow the policy, on a "take it or leave it" basis. Violating a price-suggestion is not directly actionable, but can legally result in a "we won't deal with you anymore" response. Antitrust issues arise if manufacturers conspire to get a result, or if retailers conspire. On the face of it, forbidding importation to the US except through a specific channel smells strongly of anticompetitive behavior.