It's an isolate and destroy campaign. Hiring students and ex-inmates (who will both probably tolerate more exploitation than average employees without speaking up), does not impact Amazon's bottom line nearly as much as unionized workers would. If enough people wind up thinking Amazon provides a net benefit to the communities it operates in then it will help silence those who want unions, tax reforms, or just less corporate welfare.
The teamsters don't look bad so much as they appear to have been badly outplayed by a smart opponent.