As much as I disagree with ground agents giving these seats to pregnant women (in need of space) people with young kids (believe me, it happens all the time) or leaving them last, so to be filled by the last people who check-in.
9 times out of 10 you will find that the pax will request these seats via their travel agents. As travel agents are on commision, they dont care who they sell the seats to. The airline will then allocate these seats to the pax. So if one or two do slip through at the desk then yes we are to blame but please remember to shout at your airline aswell for allocating those seats in the first place.