Did anyone notice that even when you are playing Honest Hearts, you can still listen to the Sierra Madre radio signal? I wonder if this is because, the developers forgot to have the station removed from the pip boy, or if the Sierra Madre is relatively close to Zion, or if the Sierra Madre's radio signal is really that powerful?

It's just that powerful.

It is just that powerful, after all the signal is coming from the Sierra Madre.