I think that the issue of customer contact is a red-herring. What caused the issue was not that Sam contacted the customers, but that he cc'ed his threatening e-mail to the vendor to the vendor's customers.

Any reasonable person would expect the vendor to take umbrage to being threatened with press coverage in front of his customers. Sam would have had the best chance getting the problem fixed by sending a separate e-mail to the vendor that did not include a presumption of bad faith.