The idea that someone does this willingly implies that the user knows the implications of their choice, which most of the Fediverse doesn't seem to do

The terms of service for lemmy.world, which you must agree to upon sign-up, make reference to federating. If you don’t know what that means, it’s your responsibility to look it up and understand it. I assume other instances have similar sign-up processes. The source code to Lemmy is also available, meaning that a full understanding is available to anyone willing to take the time to read through the code, unlike with most social media companies.

What sorts of implications of the choice to post to Lemmy do you think that people don’t understand, that people who post to Facebook do understand?

If the implied license was enough, Facebook and all the other companies wouldn't put these disclaimers in their terms of service.

It’s not an implied license. It’s implied permission. And if you post content to a website that’s hosting and displaying such content, it’s obvious what’s about to happen with it. Please try telling a judge that you didn’t understand what you were doing, sued without first trying to delete or file a DMCA notice, and see if that judge sides with you.

Many companies have lengthy terms of service with a ton of CYA legalese that does nothing. Even so, an explicit license to your content in the terms of service does do something - but that doesn’t mean that you’re infringing copyright without it. If my artist friend asks me to take her art piece to a copy shop and to get a hundred prints made for her, I’m not infringing copyright then, either, nor is the copy shop. If I did that without permission, on the other hand, I would be. If her lawyer got wind of this and filed a suit against me without checking with her and I showed the judge the text saying “Hey hedgehog, could you do me a favor and…,” what do you think he’d say?

Besides, Facebook does things that Lemmy instances don’t do. Facebook’s codebase isn’t open, and they’d like to reserve the ability to do different things with the content you submit. Facebook wants to be able to do non-obvious things with your content. Facebook is incorporated in California and has a value in the hundreds of billions, but Lemmy instances are located all over the world and I doubt any have a value even in the millions.