I know beancounters are detested here, but how much did this diversion into MIA cost United? There would have been a landing fee, perhaps the MIA per passenger departure fee as well, the cost of additional fuel, and whatever charges and fees were levied by the airline whose gate and services were used to disembark the offending purser (United not having a station in Miami).
According to the New England Journal of Medicine, the cost of a medical diversion can run from $3,000 to $100,000 (in 2002 dollars). Airlines are charging disruptive passengers for the cost of having to divert the flight and toss them off the plane, and the chaege is running in the tens of thousands of $. From a precedent standpoint, United would be ill-advised to indicate that this diversion was at little or no cost, given the precedent that might carry when they try collecting in the future from a disruptive passenger.