I think a lot of it has to do with that last part of your comment. The amount of times I've gone to the grocery store to find there's no register open other than the self checks or that there's 1-2 open at a huge grocery store with a 6-8 people in line for them and no self check line... People are being forced into self checking when they don't want to. These people are obviously going to be more easily upset by issues with the self check machines. Walmart in my limited experience (try to never buy anything there if I can avoid it) is the worst offender I've seen.