That's just the way the internet works: if it's on the internet, it's public. If it's "done in public," it's public domain; copyright notices notwithstanding. Many content providers put restrictions on the ability to download content or readily share it, in some cases, including paywalls. OTOH, many content providers encourage sharing on social media because "it drives traffic," and advertising pays the bills.
Anything marked with a copyright should clearly be respected. As responsible stewards of genealogy data, we here at WikiTree are obligated to make a concerted effort to not violate or take advantage of un-attributable sources, and to document the same.
Find A Grave, particularly is an open site. As a matter of standards, though, like I said before, we wouldn't want to regurgitate posts from Find A Grave, especially if we have no confidence: a photo without a source indicator is basically worthless.
Our goal is to make our work accountable: to document sourcing to the extent possible so that others can verify or reproduce how we make our claims. Without that level of validity, we are unreliable as genealogists. Too much fake work exists, already, so we should joy in making our product trustworthy. Thus the emphasis on sourcing. And, to repeat myself, that process of sourcing actually gives credibility to the extent that we give credit: so it's a two-way street of celebrating our joint findings.
