I think it is necessary for the forum that users do not have the right to ask that all of their posts be deleted. While I sympathize with a user who feels the host has wronged them and they want to take their marbles and go home, Inventables has to protect itself against the damage that this would cause. Posts and replies are not in a vacuum, so if you pull your posts then you also lose the context of all posts that reference or reply to it. Imagine if certain very high profile posters here deleted everything they’ve posted. Even worse imagine if any of them approached Inventables and said “pay me or I delete my posts”?
The terms of service must include a nonexclusive license to keep the content posted here, but do so in a way that avoids the mistakes if companies like Makerbot/Thingiverse which were perceived as taking ownership of others intellectual property.