Zell Miller wasn't exactly getting primaried either back in the day eh? It has to do with pragmatism. The only person they want to punish is Cooper over in TN who railed against the president on the bill, acting all blue dog hardcore when his district is Democratic. Cooper changed his vote the second time around, much like Boyd did.
Miller didn't stand for reelection... and I'm pretty sure a lot of people would have wanted him primaried had he chosen to stand for reelection, even rational Democrats.
I agree wholeheartedly that the Democrats are better at being a "big tent" than the Republicans, but the Democratic coalition is pretty fresh and just happy to be in charge, while the Republican coalition is old, stale, and resentful.