I think it is definitely an each to their own kind of thing, while i don't think i could have done it i can see why someone would try it.

DD2 could be VERY vocal about her want for booby (just ask HotI) and had we had major issues weaning then it not tasting so good might have been my only way out.
I don't think it's cruel, it's another way of weaning.
Just like telling your child there is no milk (when there is), that it tastes bad, that your boobs are sore - all are things that stop the child from getting what they want, all will cause them to be upset at the refusal.

Each to their own, as long as someone is not using actual poison then i don't see the issue.