Yes I know that's an ideal world etc. But how did people handle that before there was email? That's right, they didn't, and it had to wait until tomorrow.
This is not a new phenomenon.
Edit: Just wanted to add that I'm not trying to justify this behaviour. It was wrong to intrude into people's personal lives and expect unpaid work then and it's wrong now.
Of course, but for the vast majority of cases the right mental model is "too bad". Whoever needs you to be available after hours needs to learn how to plan and execute his work. Emergencies happen, but those should be rare; if not, wean them off or look for another employer. Being on call all the time is not healthy long term. Today's job market is very friendly to engineers; if your company does not minimize those emergencies, consider working elsewhere.
Yes while the employer may not be able to legally force you to handle emails outside of official work hours and doing so counts as working...
Depending on your job, if you don't check and reply to some time sensitive emails outside of work hours, you might a) become a roadblock on someone else's project that's due soon b) be late to respond to "emergencies" and this can affect how people perceive your overall effectiveness and performance.
Of course, company culture can minimize last minute requests, unrealistic deadlines, etc. to minimize these "emergencies" that need quick replies but many don't.