In the case of LHR T2 specifically, they kind of bring it on themselves by putting a shop full of alcohol and perfume right by the exit door and filling the space with Heathrow Express ticket sales people (who are also a few meters away on the other side of the door, so it's pointless to have them in this corridor). People might walk right past them then suddenly think they have to buy something and just totally ignore the fact the door has a big no entry sign on it. It's a stupid reason yes, but LHR could just put staff there who have to watch out for people going back and deter them from doing so. They won't get back into the baggage hall though as there is another set of doors which lead there and they are monitored by customs personnel. The most they can do is buy a bottle of Vodka or a train ticket which is all extra revenue for LHR, so perhaps they don't really mind.
As for people ignoring barriers, well that's just their own ignorance.