"stiffing their workers"
United Airlines has pension debts, i.e. money to be paid to workers in the vicinity of 9 billion dollars, a large chunk of which the taxpayers are going to be cleaning up. 5 million bucks is about 1/1,800 of the amount flowing to the union members, not counting health and other benefits.
I'm not justifying these bonuses, but they don't make much of a dent in the problem if you eliminate them.
The unions have delivered enormous benefits to their members for many years, and the unionized corporations would be breaking under that load even if the executives were getting minimum wage.