I’m not convinced that pay and bargaining is what Tim Cook was referring to. He is talking about deliberate IP leaks to journalists, competitors, or foreign governments. Most of the leaks are from Apple’s supply chain, from contracts that had to legally disclose some information, or accidental leaks through source code, but there have been a few notable internal leaks which were significant around the time the letter was penned. I don’t see how that letter violates laws.
It is true that there have been documented cases of lower level managers that violate employee rights and there have been unlawful terminations over discussing pay. I think you will find some of those violations in any large company partly due to incompetence of managers that improperly apply rules. Particularly in tech where they may not have been hired for their HR skills. I don’t recall noteworthy cases of improper firing in the last year, so I feel Apple has learned and improved in following the law since some of those cases.
Union busting is a more deliberate reason and I’m sure higher level execs have participated in condoning that. Companies like Apple are afraid of losing executive control over their companies almost to paranoid levels. Although Apple might take it too far sometimes, unions may also take things too far and interfere with a companies right to form contracts. I really don’t think either side is purely good.