Why, at the supermarket checkout, are some people reluctant to put their shopping on the conveyor belt behind yours without a having plastic divider to separate the shopping?  Are they afraid I’ll maliciously buy all their shopping and take it home?

Their solution seems to be to create a Grand Canyon sized gap between my shopping and theirs. 

Perfect.  It makes more sense than simply adding a divider once one becomes available.

Also, what about when it’s a ‘self service’ checkout? 

I’m pretty confident that I’ll know when to stop scanning my items.  I won’t accidentally buy your tampons, microwave cheeseburger, budget bread and extra large condoms.

Oh wait, those last ones are mine.