They put that date on there for a reason. It's probably still good, but maybe not as good as it was a few months ago. The store owner probably just doesn't have any way to return older beer, or doesn't know that some beers are better young. (or doesn't want to mess with returning it.
I believe the beer store I frequent sets their older beers in a certain place for the distributor to pick up and take back. I've seen some pretty good names sitting separate in the back and wonder what they are doing back there.
My keg of IPA from a craft brewery is still good enough to drink (now that I am over my sinus infection) but the hops aroma is practically gone, and that is a very distinctive aspect of that beer. Just drinking it to get rid of it now.
I prefer a bottled on date, so I can see how old it is, but many people might not know when the best age is to drink certain beers.