A collaborative corpus adopts the "default copyright" if it does not demand any explicit licenses for contributions to it. In other words, contributors retain copyright for their contributions, and by the very act of contributing are assumed to give some sort of non-exclusive license for redistribution. On wikis, it is often assumed that the act of contributing also implicitly gives a license to create derivative works (to refactor) on that wiki site.
MeatballWiki is a example of a wiki which adopts the DefaultCopyright. MeatballWiki is sort of a strange case, though, because there is a page MeatballWikiCopyright to clarify the issue, which might be taken as some sort of written license should there ever be a court battle.