There isn’t implied consent to share when you upload something to the Internet. However, it is clear that people have forgotten, or have not learned, how the Internet works. Everything you put on the Internet will be duplicated and shared with others, and you will have little say in how far it goes. By nature of uploading it to the Internet, you lose control over its spread, even on the nominally “private” Internet, which will eventually leak.
This is what we teach the youth early on: be mindful of what you put on the Internet, because you can’t go back. People should know that anything they upload to the Internet will be copied over and over again, that there will be nothing they can realistically do to prevent or undo it, and that they will have to tolerate the consequences. If that is too much to bear, then simply don’t share it.
This is why you will see media multinationals avoid uploading their whole works to the Internet until they’re comfortable with the prospect of seeing them copied. Movies generally come out in theatres first, then are released on digital and physical media much later, once they’ve been in theatres long enough. However, camrips still happen, because the analog hole is inevitable.
When they do have to deal with a digital release, they use DRM, region-locking, and all manner of obstacles to prevent the general public from freely accessing the content, but even they know this doesn’t stop the media from being copied, which is why there is RIAA lobbying and entire law firms dedicated to enforcing IP rights. But this only works for the biggest players, and it requires an enormous stack of cash.
If you don’t have entire organizations dedicated to protecting your work and stacks of cash to enforce your IP rights, I would recommend simply not sharing what you’re not comfortable seeing shared with others. It sucks, and people should be honourable, but it comes with the turf.
A somewhat related phenomenon is people being disappointed to see their public Discord messages shared, captured, or indexed by search engines. This disappointment comes from the imagined existence of a “middle ground” category of privacy wherein the data that you put in public channels isn’t actually public but rather “implied private”: public to those in the know, but private to those who aren’t.
Here’s how it actually is: there is public data and less-public data. There is no such thing as semi-public or private. As soon as it’s given to others, whether a person or a company storing that data, it’s no longer private. It may be concealed for a time, and you may transform it with encryption so that you need a passphrase or some other token to read it, but at some point it will come out.
Messages that you post in channels that anyone can access, even if that access requires some hoops or special awareness, are public. They’ve never been private. Therefore, vigilance is important when it comes to everything that you send. Can you tolerate this message being captured and sent to others? Are you aware that anyone can join the community you’re in? If not, then please do not send that message.