The stamps just proved that you sail was measured and approved at that event. Some events just have the measurer sign your sail, some just check you into a database, some expect you to comply with the rules and have a class-legal sail but don't even look at it. Yes, it seems that only some of the bigger events measure sails.
FYI, "measuring" a Laser sail is mostly about having your sail numbers/country code in the proper places. This isn't a problem with the folks who are regular big-event Laser sailors, but is an issue for events that bring out local-only, casual, or junior sailors.
I helped with measurement at CORK a couple years back. Many of the juniors and their parents had no clue that 1) they needed to join the class association to race (most didn't know their was a class association) and 2) that they needed to have class-legal sail numbers.