The enrichment class may be something he views as fun and thus they felt taking it away was a punishment. Personally, I would have made him sit out the whole next recess instead of taking instructional time away. Did you let the teacher know you did not approve of their choice and offer other punishment ideas?
It's important to note you said your kid was Kindergarten age. Kids that age aren't evil masterminds. Bullying to me is more of a systematic or pattern of behavior. Not a one-off hairbrained idea to swipe a hat. Particularly if you've not had multiple discussions about him being unkind to others on purpose.
Maybe the teacher was hoping you'd be the one to show gratitude and not your son. I haven't found many kids who are grateful for school work, home work, or sheets upon sheets of mind-numbing busy work. Regardless of their ability! And particularly at that age.
