In all fairness, I don't think it's that big an issue since you're providing free traffic/exposure to their website. I'm not sure why would anyone have an issue.

If you go by paper, lot of things can be an issue but that's what happens on all social media. People copy/paste and share.

You will need some very strict moderation to watch every single thing that gets posted. Not something doable for any website (including reddit which is one of the biggest platform).

If someone is sharing a content that can be helpful to others (assuming they're providing source link), I don't see a reason why they shouldn't get up vote.