Willow, I cannot speak for the activities of these teachers specifically, or what the United Federation of Teachers is having them do, but I can say that as a Union Steward and as someone who sat on the Negotiating Team for our last Contract, there are times when Union business does need to be carried out during the regular workday. I missed probably an average of part to all of one day every two weeks for Union Business between January of 2009 and March 31, 2010 related to the negotiations for our contract.
Without the ability to do certain (not all, but certain) business during regular work hours, Unions cease to exist as a viable entity. Whether it's negotiations, grievance hearings, etc... some of these things just cannot be done after hours.