People generally don't have a problem cutting or picking flowers because they don't think think plants suffer and feel pain like animals do. That's why it's not considered murder to eat fruits and vegetables. Maybe it will be someday. People will be protesting at produce stands and trying to set the salad free in restaurants... or maybe they can't feel pain since they don't have a brain or nerve endings.
Anyway, people can always get a potted plant if they don't like to hurt flowers. My mom got one for my grandmother on Mother's Day. It came in a large coffee mug that surely will last a lot longer.
Also flowers are usually considered pretty and attractive. But people kill weeds all the time without thinking about it. And apparently wildflowers are really just "pretty weeds". Flowers are not that way just for show however, it's also to attract insects and other creatures to pollinate them. But yeah, if someone is allergic to plants it's really not a good idea to give them one. I'd be surprised if people still gave them to hospital patients because you're not supposed to wear anything scented at the hospital, not even deodorant!
I never get flowers for gifts but that's okay since they die in a week or so, anyway. I'd rather get a stuffed animal, even though I already have a ton of them.