DAV:resourcetype for an activity

Section 13.1 of the spec says that the DAV:resourcetype of an activity
must be DAV:activity. This would seem to disallow an implementation
where the resourcetype was, say:

<resourcetype xmlns="DAV:">
  <activity/>
  <collection/>
</resourcetype>

Is there a reason for saying that an activity's resourcetype must be
(rather than include) DAV:activity?

-- Alison.