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It's not a good idea to use skin irritants in the public facing side of your restaurant, though there's no law against it, so they possibly do. More likely they'd use sanitiser which is usually milder, but still effective. There's many reasons that even coating the thing in bleach five times a day might not tackle the root problem, as I say, if there's a bezel or gap somewhere that can trap dirt and bacteria in it, then you've had it - it's the reason that in kitchens everything needs to be smooth. If there's a place for bacteria to 'hide' in those machines, then you're fighting a losing battle.
There's a couple of other possibilities, that they do clean it but not very well. Whatever cleaning solution they use will have a contact time where you're supposed to spray it on and leave it for x amount of time for it to be considered effective. Sometimes this is 30 seconds, or a minute, but I can certainly imagine a McD's bod just spraying and immediately wiping it off because they're in a rush/can't be arsed.
The other possibility is that they're just using glass/surface cleaner on it instead of sanitiser, which won't do much to kill anything other than unsightly smears. I have to imagine that McD's head office, who will produce a 'what spray to use where' datasheet, wouldn't have missed that one, though, but stranger things have happened.
It's entirely possible it's just neglect from employees, but like I say, they do seem genuinely good at cleanliness, though maybe it only extents to the kitchen where they stand to lose the most public face. These fast food chains have a huge amount invested in making sure they never get articles like this written about them, so it's very surprising that the biggest one of all got one.