As a member of one of the strong European Unions (not as strong as people might think), I'd say that the purser accepting a bribe to upgrade someone wouldn't have a leg to stand on if he were caught and sacked. Unions are not against wrongdoers being disciplined - especially in situations where co-workers have lost out through the wrongdoing - but they do ensure that proper processes are followed and that workers who are accused of wrongdoings have access to good advice and a fair hearing.