Playing devil's advocate here only a little bit, why not? Why can't union business be done during non business hours? Shouldn't teachers participating in union negotiations be doing that on their own time and not during the taxpayer's time? I have done a ton of voluneer work and also participated on employee committees and it was always on my own time, not my employers' time.
Lets take a look at the situation I deal with. I work for a utility company with workers in Rhode Island, New Hampshire, and from end to end in Massachusetts. To bring everyone together in a common location would require at least a 2 hour one way drive for many of the people. We are a shifted workforce. In many cases the people who need to get together do not necessarily work the same shift, in terms of hours, days, or both in some cases.
In terms of negotiations, this is what you seem to be suggesting..... We all get off of work around 3:30pm. We all travel to the negotiation location to meet with the company. We get started at 6:00. We negotiate for 6 hours, leaving at midnight. It takes some of these people 2 hours to get home. Then they're expected to be at work by 7:30 the next morning. Do you see how ridiculous that is to suggest?
Most of our internal Union business is carried out off-hours. However, if I have to have a meeting with my supervisor, his boss, the Union VP, and a member of the company's Labor Department over an issue of something the company is doing wrong, why shouldn't we be doing it on the company's time. They made the mistake, they can pay the price.