It's true that he should have contain, but this does happen. And it happens at a decent rate.
* This is the only one I could think of off the top of my head that demonstrated this perfectly. I'm sure if you google Vince Young you'll find plenty though.
1:34 -
http://www.youtube.com/watch?v=MD8qS3RGEu4
Now, if this happens 50% of the time, then yeah, that's going overboard. But I'd say probably 1/5 - 1/7 times the zone read is run in real life the DE will lose containment. Just depends on how well the RB is running the ball off the play. After all, if your teammates are stopping the run just fine then there's no reason to go after the RB. But if you're getting gashed time after time by the handoff, it gets really tough for the DE to stay at home.