I've occasionally been delayed by Starbucks. I'm usually a very punctual person, but there has been an occasion or two when I just needed that double tall latte before a meeting, and stopping to get one has caused me to be late (and thus inconvenienced others). So while it does seem inconsiderate if this FA delayed a plane of passengers, for certain things, such as a caffeine fix, I can understand if it happens on occasion.
(Of course all of this is easy to say in a discussion like this. I imagine that if I were one of the passengers, I'd be POed too.)