Why is it unreasonable? This happened when I was a kid and as a result most pen tops now have holes in them. We don't need pen tops either, there are those clicky pens as an alternative. If banning pen tops saves just one life then it's worth it imo.
Yup, I still remember when you could get biros whose lids didn't have safety holes.
I can't see this happening though. Alright, biros don't really need lids, but they do stop you getting ink all over everything. What about fountain pens? When I was at school most people used a Parker fountain pen. The lids on these are swallowable and don't have safety air holes, but are kind of necessary. Do we ban these too?
Lots of things are dangerous if misused, and it seems to be part of our current culture that if you get injured whilst misusing something, then it is the fault of the manufacturer for not telling you not to do it, and not yours. Most people are quite aware that chewing small bits of plastic is a choking hazard - should we ban all bits of plastic of a certain size? My feeling is that if you can't trust yourself not to chew the lid of a pen, then you can get yourself a pen without a lid (or even throw the lid away).
Whilst I feel for the parents, this was an accident caused by their son sticking something in his mouth which he should not of - not an accident caused by someone supplying him with that item.