It's either a genuine grievance or not, and given that many in the PLP are just as enthusiastic about EU membership as folk that voted for Corbyn, I'm not sure why they're unable to feel the same way.
And let's not put this down purely to the shadow cabinet acting out, either. It was started by backbenchers tabling a no confidence motion, which was expected to get support from well over 50% of Labour MPs in the secret ballot on Tuesday, meaning Corbyn would explicitly no longer carry the confidence of Labour MPs. Corbyn pre-empted this by sacking Benn, which then triggered the resignations.
The key point in all of this is what Watson mentioned in his statement earlier - the Labour party needs to be ready to govern immediately. We are potentially months away from another general election. Therefore the question that most people expected to be able to dance around until 2020, namely whether people seriously think the public would vote for Jeremy Corbyn to be Prime Minister, is now paramount. Now, I can understand when people will respond to this by saying there's no-one else that looks like a PM in waiting, either. But I can think of a few that could stand at least a chance, and could at least prevent a grown majority. Either we go for that chance, or roll over and let Boris camp down for the next few years.