@daviding is spot on, and this default copyright situation has disastrous consequences. What really would be the “open commons” here that invite, legally permit learning? There’s a reason why the Wikipedia and Federated Wiki could never work if it weren’t for a libre-free license. With the question of the original post, if insisting on libre-free licenses encourages more contributions or discourages some from contributing, well, what’s your main goal/priority, is it accumulating contributions no matter what, regardless of digital ethics and legal usability of the result? Some people indeed don’t really want to contribute to the commons, they think that in the digital world, they can maintain/demand some effective control over what others can do and can’t with their work just as in the good old print industry days (but not before), after they shared it with absolutely everybody and given that cheap electronic copying devices are on every desk and in every pocket. The US Fair Use law doesn’t apply in my jurisdiction (laws are only always national), and honestly, I think it’s a bad change to begin with, because it avoided a sound copyright reform and instead added more special case exceptions, for which no-one can have any legal certainty, of how courts would decide what uses are considered fair and which aren’t, as it’s actually much narrower than most people realize. Creative Commons BY-NC-SA isn’t a free license, because of the NC non-commercial clause, as free licensing isn’t against commerce, instead invites every co-contributor, everybody from the community, everybody outside (general public) to make a (non-exclusive, non-restrictive, therefore more ethical) business out of it to the best of their ability. A copyright notice at the bottom indicates to the reader/visitor that certain rights have been granted (and others carefully retained), but the actual mechanism is to have an agreement on the edit + upload page that requires the user to only submit if in possession of the rights and if consenting to grant them. You as the operator are also liable for copyright infringement done by users of your services, so you better have something in place (which is likely already the case) to make sure that you’re not knowingly complicit in the violation of the copyright demands of others.
Another difference in regard of the activity here and in chat is that a chat allows for better “curation” in terms of directing messages to a single individual or a channel/room with members who share the same interest, and it’s a little bit harder on forums/boards to manage notifications and federate messages, to not spam/pollute everybody else’s reading with posts that are likely of no interest to them. Also, private messages are hardly ever of concern in terms of copyright and the commons, because despite they’re surely copyrightable and might sometimes be interesting content-wise (and keep in mind digital posterity/legacy), they’re not sent to a distribution channel for publication, and addressed to only one receiving party, usually on the assumption/expectation that the content remains confidential/private (which might be why there’s no reason to bother to replace the restrictive, exclusive “all rights reserved” default for them), also phrased/written specifically to the particular audience of the single recipient reader, and while that too doesn’t grant at all any effective control over what people can and can’t do with it any more, it’s also clear that the original author didn’t intend to share it with a wider audience and publish it for the general public to read, so it shouldn’t be assumed that such exchanges can be part of a generally shared/stewarded commons. Even in this thread here, we’re in dialogue with each other, not necessarily directly addressing the random readers of the public unknown to us and optimizing for them, so it’s always nice and useful to have this licensed under a libre-free license, but it’s not necessarily useful on the content-level, or the exchange of opinions or works of art/entertainment don’t necessarily need the same digital rights granted than works of practical use do, but who knows, this is already pretty much converging on the latter. If it’s not copyright versus software, you may consider https://www.youtube.com/watch?v=eginMQBWII4 for other types of works, and of course https://archive.org/details/StealThisFilmII