You may not have heard of this, but a union of companies is called a "trust", and there's an entire division of the Department of Justice devoted to preventing them from forming, and destroying them when they do arise: http://www.justice.gov/atr/
Congress has actually enacted specific legislation to exempt the MPAA and RIAA from some aspects of antitrust law. I don't know about the BSA.
To be more specific, though, there are some ways that companies can legally cooperate under current antitrust law — for example, standardizing screw threads and grades of steel. But boycotting the "products" of certain "vendors" is probably not among them.
