pushy salesperson

Something very related to my previous post happened today.

I was in my bedroom. I've locked my house's main gate, and the wooden door was closed, although I didn't lock it. Then suddenly I heard clicking sound of the door closing. If it was already closed, how could it close again? (I may seem deaf at times to human voice, but I can pick up certain frequency range better)

So I went out to the dining room to investigate. True enough, a man was trying to push open the door but there's the door-closer installed on my door. I pushed the door shut and asked what the man wanted. He was promoting those weld-on holder for extra locks, and cited something about some unit in the neighbourhood. Blah blah blah.

I didn't think salespeople have the cheek to turn the door knob and open people's main door. What next? Battering ram?

(and so, it makes sense to install door-closer in residential apartments too)