[Zope-PAS] Installing Plone PAS in Plone site overwrite base acl_users and sites acl_users

[Tom Hallam]

Just tried PlonePAS on a test site (clean install 2.1.1 on current Zope 
2.8.x) and was getting an assertion error when using quick installer. 
I'd done my normal setup of using a different user name and password for 
the zope admin account and the plone admin account.  Just to try 
something else ... I decided to log into the plone site using the 
acquired admin account from the Zope install -> No error but it 
overwrote both the Zope and Plone acl_users.

Is this normal for PlonePAS?
Why?
Does it mean that all plone sites on a zope instance must use PlonePAS?

Tom