WebStorage and WebDatabase - creation and exceptions

In WebDatabase:

The user agent may raise a SECURITY_ERR exception instead of returning  
a Database object if the request violates a policy decision (e.g. if  
the user agent is configured to not allow the page to open databases).

In WebStorage (emphasis mine):

When a new HTMLDocument is created, the user agent must check to see  
if the document's top-level browsing context has allocated a session  
storage area for that document's origin. If it has not, a new storage  
area for that document's origin must be created.

When the localStorage attribute is accessed, the user agent must check  
to see if it has allocated a local storage area for the origin of the  
Document of the Window object on which the method was invoked. If it  
has not, a new storage area for that origin must be created.

A browser may not allow local storage for a certain origin, just like  
it may not allow cookies to be stored. What is the expected behavior  
in that case?

Alternatively, why allow browsers to selectively block out WebDatabase  
and not other kinds of storage?

Nikunj
http://o-micron.blogspot.com