The username token profile specs are not really clear about this, anyway this was not addressed the proper way when working on JBWS-2265.
It's a bug, I've created an issue for it: https://jira.jboss.org/jira/browse/JBWS-2309. This will go in the first release coming out.
This issue is fixed on trunk now. It would be great if you could build from sources and give it a try and see if you can correctly talk with the .NET impl now.