"It was consistently shown that the condition in which the supervisor was “always” included by cc made the recipient of the email feel trusted significantly less than recipients who were randomly allocated to the “sometimes” or “almost never” condition," said David De Cremer, one of the authors. 

A further survey of 345 workers came to the same conclusion, and further found that when supervisors were copied often, employees also felt there was an overall culture of low trust in the organization, which in turn creates an atmosphere of fear and low psychological safety. What's more, those who do copy their supervisor in an email are well aware of the signals such an action sends. So, apparently, people on the receiving ends of these emails feel less trusted because, well, they're less trusted. 

The fact that these results came from both western and Chinese samples of employees suggests that this is true even in cultures that are very different.