Well I think because some people are unbalanced. I can recall a woman posting in the Cherokee forums by the name of Neni or something like that was had serious problems and made up lies. I think she was the moderator of the forum. After reporting her to Ancestry I made my profile private for a while.
Besides, if they researched it you can and often their research needs correcting. I still see trees with mistakes I made in the 90s in other trees that I've since corrected, not that I don't have other mistakes to be corrected as best as I can research.