As lots of people mentioned, CBS and FOX can cross flex games. The reason it was done this week is probably because FOX has the NYJ-SF game, and the 49ers were the only team of the 4 expected to be good, & FOX wanted to keep that one over the Giants.
What I found most interesting is if you click on the map link it says that Utica switched from NYG-Chi to the Bills game. Pardon my ignorance, but I thought the Giants were popular in NYS east of Syracuse.