So, I wouldn't put this on my car (also breastfed 2 kids). But lets play devil's advocate here.
The idea of these stickers is that if there is a bad accident, you would want emergency services to know a small baby was on board, so the first priority is to find a baby.
In that situation, come to think of it, it might be useful to know if a baby is breastfed (so very small and possibly will have difficulty feeding from a bottle).
There have been accidents when the parents were in the front seats and have been seriously hurt or killed, while the baby in the very safe car seat at the back had little if any injury. In that situation, there might be no one about to explain that the baby wasn't used to having a bottle etc., and this would be important information for a tiny baby.
On the other hand, this is not a very likely this to happen, thank God.
I wouldn't assume that this was someone with whom I could not be friends.
But I would wonder whether it was a first baby 