From what I understood the union bit was only part of it. AOL and Yahoo came from the Internet content world and Email was their way of driving people to the content, which included ad revenue. Verizon Email began as and operated as a service, which Verizon customers got (”for free”). Verizon Email, from the web service end, was an “IT” service internally at Verizon, which gradully added some content folks to get some ad revenue into it.
So from Verizon’s point of view, they needed Email to move to their best “content providers” and that was their subsideraries AOL/Yahoo, not their internal IT folks.