The guns are not sterile. Staff at Claires get a couple of hours training, then pierce each other. The aftercare advice is complete rubbish (harsh chemicals on a fresh piercing and twisting the jewellery is a massive no-no!), and they're not allowed to turn anyone down, they pierce through bumps because they don't get trained to recognise them and that they're supposed to avoid them. You're very limited in the placement with guns as well, so if you have very flashy or long lobes, they can't be centred. I actually hate Claires with a passion...i'm trying to organise a bit of a protest outside our local one, and hand out flyers with advice on and money off vouchers to get it done at a proper piercers nearby.
the guns themselves arent sterilsed , but the earrings are sterile. i thought it was impossible to fully sterilise piercing guns anyway which is why needles are considered to be better?. the training involves basic anatomy of the ear, a demonstration video, practice on using the gun on without piercing before the trainer says yes or no to them practising on another human. not everyone who does the training ends up passing it. they are allowed to refuse to pierce if the lobe isnt suitable , they refused me as i have thick scar tissue in one lobe from an old piercing.
as for aftercare, i've always been told to gently clean then rotate the piercing. the assistant in claires who did my friends daughters piercing told her to gently wiggle the stud after she cleaned it and did say that, although it was company policy to provide cleaning solution with each piercing, she might find a mild antibacterial soap or saltwater better
i'm not disagreeing with you that obviously claires will have less training than a reputable piercer with years of experience. but at least around where i live, there are several places that will pierce ears without taking as much basic care as claires. and again, as you said, a decent piercer shouldnt make you feel uncomftable asking for any piercing, but there are some places that will make you feel uncomftable . presuably, the standard given in each claires store will vary anyway so maybe i've just been lucky with the piercings i've seen done by them
fwiw, i have 3 piercings in each lobe. 1 done by a needle in a not very hygeine peircing place , while i was drunk

which not suprisingly became infected, so i let them close up, leaving a scar. 1 was from claires, which healed fine, 1 was in a local hairdressers with a gun which healed fine, and the last 1 from a place in brighton ( near wizard of ink, but i forget its name) who were brilliant, but i was getting my tragus, nose,lobe and labret all re-pierced done in one go, which took ages to heal well but that was more to do with me constantly catching it on earphones and clothing