I think I found out what happens.
For starters:
1)Go to a website that you have an account for (say,
www.technorati.com)
2)Open up PM
3)Make sure that the treepicker shows the URL to match (not necessary, makes the bug easier to spot)
4)Open the account settings for the Default Options
5)IMPORTANT! Click on either the URL to match or the URL to use field (should now say, for example, "technorati.com")
NEVERMIND!! Ignore number 5!! Just click 'Okay'6)Close the account settings dialogue [EDIT]Using Okay[/EDIT] for the Default Options and LOOK AT THE URL TO MATCH. It should now say technorati.com
7)Sure enough, you will be prompted to choose between the default options and whatever account you're using.
This proves that it actually can be used on any site.