A “scab” in an employee that comes to work during a strike, whether newly hired or not.
“Crossing a picket line” is when workers, scabs or otherwise, come to work while there is a strike going on.
“Boycotting” is when customers and consumers refrain from buying products from a company or corporation.
People are getting mad at customers for going to Starbucks. That's counterintuitive thinking. You need Starbucks to be flooded with customers, to prove that the company needs its employees. You should be mad at the scabs. Not the customers. Generally speaking, when a boycott occurs, layoffs follow.
Hope this helps, good luck out there.