Welfare recipients DO have people's lives in their hands... the ones with kids obviously, but if nothing else they have their own life in their hands and suicidal people are monitored, if needed by force, to protect them from themselves... but also even with no kids they have other people's lives in their hands because them being on drugs can cause them to harm others... whether that's assault in a rage, robbery to fund the habit, or crashing because they're skitzing out those are all very real threats to innocent people.
As to grocery clerks... what's the latest numbers on the amount of salmonella, e coli, etc in foods? And what happens if you don't pay adequate attention and allow your store bought chicken, beef, pork, etc to come in contact with your carrots... carrots that are not cooked, just given a quick rinse, cut up and fed to your kids? So yeah... clerks DO have lives in their hands. If they're stoned they aren't going to give a crap if they put a leaking package of (quite likely contaminated) chicken in the same bag as some kid's afternoon snack.
Savage you summed it up. If you know you'll get a positive test because you're a user then don't fill out the application, then act all indignant when a test is ordered, n'mind when it comes up positive and you're not handed cash. Whether that is for a job or benefits, if you know that's part of the 'interview' and you willingly choose to subject yourself to that interview, then you give up your right to privacy. Just like you give up your right to financial privacy.
Now should having a positive test ban you for life? No, I don't think so. See previous post about the background thing. That can hurt clean people. Folks who've grown up, bucked up and are doing their best. If they're clean, no risk that their kid will starve as they zone out in a drug haze or their kid will be beaten in a drug rage, then I've got no problem with lending them a hand.
But I'm not going to hand someone I know is high the money for their next hit. I'm not going to be an enabler that helps anyone hurt their self, their kids, or innocent bystanders. I don't want to be a statistic, or for my kids to become statistics, in the news as just another injury or death due to drugs. Not that I can stop users from using, they make their choice, but I'm not going to hand them the drug any more than I'd hand a suicidal person a gun. I don't think our government should either.