Why are people so protective over photo mode pictures?

I sometimes like to repost cool photo mode captures that I’ve found from pinterest, twitter, etc. on social media and some people will get angry with me and tell me I’m stealing art (even if it isn’t theirs). I’m not trying to be rude but I don’t really understand how I’m stealing anything, can someone explain how that works? I see some people watermark photos which is fine and I share my own photo modes too but I don’t really care if people repost them.