They are not responsible for what people post, nor do they pay anyone to post, therefore I do not see how they can claim the data as “theirs”.
They have their own self-regualted rules, but ultimately most anything is fair game for reddit to point at the user and say “we take no responsibility for what an individual may post on this public form”.
The only thing they will have a problem with is CSAM, but even then as long the volunteer mods remain effective at removing it, reddit will not be responsible for anything users post.