Of course children can pick up a bug anywhere - and ime mainly they do.
But to deliberately take a sick child, when you don't know whether they are infectious or not, to a party where they will be in close contact with lots of other children, sharing food, playing games etc, therefore significantly increasing the risk of other children getting it ... not exactly shocking, but unbelievably selfish imho. Yes, of course the kid wanted to go to the party - so what exactly is wrong with saying no, sorry this time you don't go, you're not well enough? There are going to be hundreds of other parties in this child's life! Why can't people say no to their offspring anymore?
Bellalasagne - I agree completely, I would keep siblings away too if there's a stomach bug in the house.
It is just tiresome, tedious, expensive (sometimes, for parents who have to take time off work to look after sick children) -when your kids are ill. They already get hundreds of bugs. Who wants to be exposed to any more?