The Articles make it very clear that it's not the fact that they paid with pennies, but what happened as a result of doing to. Dumping 2500 coins on a poor clecks desk, for them to have to pick up and count, is not only a very disrespectful/unfair thing to do, but you are also being diruptive at the clinic (which is what he got fined for "disorderly conduct"). Slowing down of the lunch line, by using the smallest coins you can find is also disruptive to others (if you read the article giving more than one example).
Legal, yes. But since these people were simply using the smallest coins possible to purposefully spite others and ended up disrupting other people's lives, they were rightfully punished.
Legal, yes. But since these people were simply using the smallest coins possible to purposefully spite others and ended up disrupting other people's lives, they were rightfully punished.