problem FOR struts is not a problem OF struts. you are absolutely correct that this is a problem, but it's not a flaw of struts. struts is a framework that structuralized and simplified your web application development. it's not set out to address every problem of basic web technologies, especially browser issues particular to the multiple sessions dependence on browser usage behaviors.
my suggestion is consider some workflow implementation in your app if you anticipate users opening multiple windows before completing an application/business path. see http://www.livinglogic.de/Struts/
for a struts implementation of workflow. honestly though, 90% of apps are not critical enough (or at least don't care too much) to enforce workflow across sessions if the users are 'odd' enough to launch multiple browsers to do the same thing.