This answer from Parse says:
You can call destroy() on any ParseObject from Cloud Code to delete them. Deleting, as well as creating or updating, multiple objects from Cloud Code is not recommended, however.
Why? The answerer doesn't say, and it seems like Cloud Code would be exactly the place to bulk update/delete objects. Is he using Cloud Code in opposition to a Cloud background job? Or am I missing some other way to delete objects in Parse?