One thing that comes to mind is the union saying that clause is a dealbreaker, and being willing to strike unless it's included.

In New York, public-sector unions are not allowed to strike; instead both parties are forced into binding arbitration. https://en.wikipedia.org/wiki/Taylor_Law

According to an arbitration decision in 2010, it sounds like the MTA strategically withdrew an OPTO clause specifically to get less hammered by labor on other issues during arbitration: https://www.scribd.com/doc/18552062/The-M-T-A-and-T-W-U-Arbi...