I have to say, although I've never done the job and have no idea about the pay and conditions of these folks, my perception is quite different.
Apart from the more eastern parts of Europe (Romania and points North and East) I've always found screeners to be very consciencious. In Prague a few months ago my Swiss Army knife that I'd inadvertently left at the bottom of my bag was carefully and politely extracted, put in a jiffy bag, and returned to me at LHR (and my bag was re-scanned after). At LAX last week every single piece of check-in luggage was conscienciously checked for explosive residue - I've certainly seen plenty of fellow passengers have bags re-scanned, personal searches when something looks not-quite-right, or tiny (and frankly harmless) nail scissors, etc. removed from their bags.
Overall, whatever they're paid, I've not seen any screeners in Europe or North America who seemed less than consiencious. If they aren't being paid a decent wage, then arguably that's unfair, but it looks like a separate problem to me.
G