It's not only vandalism. Actions like changing the licence would be hopeless if a large proportion of contributors were anonymous/unreachable. Even during normal operation, we often want to contact the author to discuss his modifications. And OSM also has a social aspect, even if not everybody is interested in it.
The need to sign up may be a barrier, but it is a small one compared to the technicalities of actual editing. Signing up with OSM is a simple action that most internet users have done on many websites before. If somebody is stoped by that... He would certainly have been stoped by the editing process later on anyway.
anyway. A proof of that is the high proportion (is that 70% ?) of registered users who have never contributed any change.
That said, there are some third-party services which lower the barrier of entry and anonimize contributors. One example is mapdust, which requires no account and lets technical-averse people contribute. Another example is wheelmap.org, which focuses on one type of map data, makes it easily and anonymously editable, and commit the changes in the main OSM DB via a "proxy OSM user".
