It's always been thus, it's just that the media world is so different now. Like, I'm not sure it's ever been possible to subscribe to NBC Sports Phllly as a cable customer and not also have ESPN and TBS and FS1 (though the reverse has often been true), so if a game is national you just change the channel. But in the a la carte streaming world this is how it goes.
I wonder if Comcast actually gets a credit on its annual TV rights payment based on how many games are taken from them for national broadcasts, or if that's just part of the deal, as it basically is here for anyone buying this product (and always has been for regular MLB.TV too).