When my Ancestry tree was public, I would get very annoyed when I saw a record or photo that stated "originally shared by" so and so - (the name of the other member). But I knew jolly well that the cert. or photo was taken from my tree, as I always add a special little mark into the corner of each before I put them onto my tree! That's one of the reasons I changed it to private.
Sadly though, I think some photos and stories from private trees still come up in a search, and people can simply copy them! I'm certainly not averse to sharing things from my tree - after all, it's in the T's & C's, and if we didn't share our stuff, probably none of us would get very far! But if another member doesn't bother to introduce themselves first, from the "send message" link, and let me know their relationship, say thank you when I say " go for it" - it's pretty poor form!
I recall writing to Ancestry once asking what would happen to my Private tree when I die, and no subs coming through. They told me it would become public.