Hmm... this is unusual. I have friends who use Hartz flea collars and their cats don't have any problems.
Secondly, there are over 75,000,000 cats in the United States. 15% of all cats suffer from at least one kind of allergy. 15% of 75,000,000 is 11,250,000 cats. Hartz says that "thousands" of cats have had problems with those collars. That's less than .08% of those who are presumed to have at least one kind of allergy.
Therefore, we shouldn't assume that Hartz products are unsafe for all cats. For all we know, most of the cats that suffered from those collars probably had an allergic reaction to them.
If you were allergic to nuts and didn't know about it, you would get sick in some way from eating nuts or food products with nuts in them. Your doctor wouldn't be lying to you when he says that it came from the nuts you ate recently. Just from that doctor's comment, you shouldn't immediately assume that the company (that made the nuts you ate recently) is responsible. It could be your allergic reaction to nuts, not the nuts themselves.
If you read the article on that hartzvictims website...
The vet said that it was definately caused by the collar.
Did that vet specificially say whether it was an allergic reaction or not? If the cat was allergic to that particular collar, then the vet would still be telling the truth. That's why the vet could mean at least two different things.
Don't jump to conclusions, go running like beheaded chickens, and cry that Hartz is a horrible company. Maybe, they are... maybe, they aren't.