You're right, that's what they usually do. I may be wrong, but for some reason I thought I remembered that last preseason for the Bills at least we seemed to get stuck at least once or twice with the entire game of the opposing teams local broadcast. Preseason local announcers are heavily biased, and I just prefer to hear the bias in our favor
In any event, I found the answer to my question, for those interested.
From the Support website:
Can I choose which team's local broadcast I want to watch in NFL Preseason Live during NFL Preseason games?
A: No, there is only scheduled to be one feed available for each NFL Preseason broadcast within NFL Preseason Live.
I'm assuming if you are not in either teams local market, you will not be blacked out of the live broadcast, even if the game is not sold out?