MN pearl clutchers are very odd on this issue. I would not have thought of them as anything different to anything else sugary and coloured, like brightly coloured sweets, ice lollies, or e.g. cake icing - which surely are totally normal occasional treats even for young children. Not an everyday food but for a special occasion, I wouldn't blink an eye. I would not expect them to have a dangerous ingredient (like for example I might check a fizzy drink can to see if it contains caffeine). I would expect something like that to come with a warning or age recommendation.
Indeed, last summer we all got one on holiday, including my then just-2yo. There was no age suitability indication, and I didn't think twice about it. (He was fine with no ill effects - but perhaps there was no glycerine in it, since we aren't in the UK).
If the 2yo had been the oldest child in the family it wouldn't have been something we would have gone for I don't think, but since he has older siblings and we got one for them, we wouldn't leave him out. And certainly I thought nothing at all of letting my 5yo have one, and the display was very attractive to him. I would imagine that lots of parents would do the same even if they don't like slushies themselves.
So yes, it's helpful to raise awareness around this because if indeed there is a dangerous ingredient then they should have an age restriction or at least a warning. And no it's not just "feckless" parents who would let their child drink slushies 
such a weird assumption - it's a bit of ice and syrup in a cup. At least, that was my assumption before I learnt of the glycerine.