I don't have a problem with the term Bench Bombing, yet it does seem to draw more of a focus on the activity of what your group does--upgrading social engaging in our public spaces. And that's a good thing. But I'm curious if you anticipate that activity to scale? It's possible the act itself could become something branded in its own identity and spread organically, like Flash Mobbing. Perhaps that's what you guys are going for? That could work very well with other groups willing to create an inventory of benches and get them into public spaces in the right place at the right time, as you've all done.
Maybe the bench design itself could be branded and named something, to draw attention to it being a central part of the activity. Because from the pictures above it does look like you guys have created a unique product that deserves its own identity and could also drive a rapid spread of these civil behaviors into public spaces as well... maybe even brand the instructions of how to use the bench on it? You could call them Busker Benches, or some-such. Just musing out loud.
Good stuff---love when groups help citizens better enjoy our public spaces. Cheers!