Regarding boycotts promulgated by cities, who are legally creatures of a State in the American federal system, they are unconstitutional as I read it.
Article I, Section 8, Clause 3:No state or agent of a state is allowed to impose a tariff or restrain commerce between the states. That is what happened under the old Articles of Confederation. If California or some city within California can discriminate against Arizona then what could stop NY from imposing a tax on Vermont to simply extort money from them or threaten to prevent state residents from traveling to spend money on a ski vacation? Private citizens can refuse to spend money and I can see a state being allowed to buy within the state but how can a state prefer some members of the Union to others?
“[The Congress shall have power] To regulate Commerce with foreign Nations, and among the several States, and with the Indian tribes;”
Article IV, Section 1,
"Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State. And the Congress may by general Laws prescribe the Manner in which such Acts, Records and Proceedings shall be proved, and the Effect thereof."
"The Citizens of each State shall be entitled to all Privileges and Immunities of Citizens in the several States. ..."
(last 2 sentences added at The Volokh Conspiracy)
(follow on comment at Volokh)
Nameless: “If I’m not mistaken, cops normally do not have the authority to enforce federal immigration laws or even to question people about their legal status — only the border patrol and related agencies can do that.”You are mistaken. Not only CBP, ICE, the Coast Guard and the Border Patrol are authorized to question people. What they are allowed to do is conduct warrantless searches at the border or the functional equivalent of the border. That means not only the physical border and airports and ships at sea but if in hot pursuit anywhere on US soil. Any law enforcement officer can be trained and certified by Homeland Security to assist in enforcing immigration law. See this ICE link from FLETC.
Note federal customs and immigration officers can do warrantless searches because until someone is admitted they are not strictly speaking in the country, so the 4th Amendment does not apply. However if not followed in hot pursuit it must be assumed that someone has those rights until proven otherwise after a proper examination, at which point they are not simply searched, as they were after a warrant was obtained when not in hot pursuit, but deported. Local LEOs trained to conduct immigration inspections would not be doing warrantless searches.