Where I live (Seattle), a portion of the self-checkout lanes are also closed to coincide with the level of shoppers. I was told that this is because of labor unions - basically it guarantees work for the cashiers.

I've never successfully used self-checkout at a grocery store without triggering the anti-theft mechanisms. I used to have this problem at CVS but I'm pretty sure they've disabled the scales in recent years. Likewise for Target--scales disabled. I assume grocery store theft is more common, or perhaps their margins too slim, to disable the measures.