Earlier quoted context omitted.
Section 230 does not talk about ownership. A freight company is generally not liable for contraband shipped without its knowledge. That does not mean that all shipments become the property of the freight company. Same thing.
A freight company that took your freight, refused to give it back, and acted like it was now theirs would appear to own it. If Reddit says "your content is yours but you cannot delete it" they're in a bit of a cake-and-eat-it-too scenario. I'm entirely sure it's legal (as other forums/HN/etc do it too) but I'm not sure it's right .
Yes, but the user is also in a cake-and-eat-it-to scenario: they get to own the content and have it distributed for free.
Both sides of this come from the perpetual non-exclusive license that posting to Reddit or Twitter or anywhere confers. "Ownership" is too reductive a concept for IP, which is why the section 230 thing is a non sequitur.