What is this strange thing with petty thefts at weddings?

I once had my thank you gift from the bride and groom for being a groomsman pilfered.

At another wedding I had the gift that was at each place setting snitched by the end of the evening.

At one a friend won the floral centre piece at her table in a contest but when she was ready to leave it was gone.

At a recent wedding the maid of honour's bouquet was stolen from the head table.

I am told by friends this sort of thing is quite common at weddings.

What goes on in people's head that they somehow convince themselves that it's OK to steal at a wedding?