I'm sorry, but this seems like a bug to me. Switching *back* to a non-layout tab doesn't disable the button either, and if it's the "last layout selected", well, that would be really confusing. ;)
To add: If I'm on a layout for project 1, and right-click and close project 2, it still goes away, even though I'm on project one's layout! Come on, that has to be a bug now. ;)