It depends, and being starbucks, I’m not sure what kind of investigation they did. But suppose they found that the servers regularly played “practical jokes” like this, and the manager had encouraged them or allowed it and made it seem like it was acceptable.
The company may have just decided that wasn’t what they wanted in a manager. She said she saw the muffin; but she didn’t then ask if they had labeled anything else wrong; she apologized, but then allowed them to give out the cups; she might have violated company policy in trying to offer the department free drinks if they came back, rather than working through the corporate legal and/or customer relations department.
It could also be that when she spoke with them later, she said something about being childish for posting to facebook, and just made matters worse.
We don’t have the entire story.