Why is 'Allow Snapshot Isolation' set to TRUE on the master and msdb databases? Why is the database property Allow Snapshot Isolation set to TRUE on the master and msdb databases, but FALSE on other system databases? The default option for this property is also FALSE when creating user databases.
I would imagine that it is enabled to minimize blocking in the master and msdb databases of the system processes.  It is disabled in user databases because in user databases it can greatly increase the disk IO requirements of the tempdb database.