The experience you're describing still doesn't need to break the back button. Going back means going back, not closing a window I never opened. If that's an awkward experience, don't build one that works that way.
Fair ‘nuff, and I agree, but would they be able to argue that they never explicitly “broke” the back button?
I remember when JavaScript became a big Web site driver. The arguments against using it to fetch and build content usually included broken back button functionality.
I don’t think a lot of folks really paid much attention to it, though.
It’s like reading an eBook in a reader. You always use the reader to interpret, format, and present the data.
It kind of sticks a spike into the old “each page is a document” model.