It wasn't so "voluntary" when you are doing it to get FCC/ Justice Department approval for a 6 billion dollar merger.
Given that, it makes sense that they'd support the notion that all other competitors be required to do the same. I don't think this is particularly altruistic in that light.
They also did this while fighting it in court. The Verizon suit which struck down the Open Internet Rules only came as a result of a previous Comcast suit.