An open source software is, by law, the maintainer's (which can be an individual, or a group of persons) property. It is said maintainer who has the right to grant you any kind of license over what he owns.
In the case of an open-source project, that license is very permissive, true, but if you take the time to read any of those, you will always see :
- A provision indicating that the owners grants that licence within the limits of the applicable laws
- Sometimes a provision indicating under which juridiction said license is granted. If not, the user local laws are the ones used.
Source : the fucking law and the fucking licenses. And my friend, which happens to be a lawyer specialized in intellectual property laws.