Earlier quoted context omitted.
I've seen several people leave at places I've worked, but never seen one of them locked out. When people are asked to "resign" I've seen them told to finish out their paid term at home, but that's obviously a lot more confrontational than someone who chooses to leave.
It happens all of the time. For many situations, there is considerable risk in letting somebody hang around for two weeks. The employee's mind is going to be elsewhere. He will be excited about the new job, and eager to talk to co-workers about the move. The employee may even decide to save some of his more valuable work to use in the next gig. The manager is almost always better off by letting the departing employee…
If someone's resigned rather than been fired or made redundant, they'll already have had plenty of time to do that, won't they?
"What I find most puzzling is why announce the resignation via email? This is almost always best done face to face, or at least via phone. Doing this electronically strikes me as immature."
I agree that handing in your resignation is best done in person, maybe it's an age thing.