I blame the unspoken expectations that come with email. Email is, in my experience, a political problem rather than a productivity bug.
Requests for support on active bugs comes through email. Your manager will request features, or ask questions about your code and status, through email. Coworkers on different teams collaborate through email.
To not answer these emails immediately probably would not lead to readily apparent punishment, nor would my manager say “he insisted” I respond that quickly, but other coworkers who prioritize these active issues, are ultimately more visible, promote faster, and get bigger bonuses.
By choosing not to play the email game, in many cases, you won’t be punished, but you won’t be rewarded either.