I mean, eagles fan get personally offended if you're not an eagles fan regardless of who you root for. My wife is a Steelers fan and has gotten shit over it.
But they reserve a special place in hell for Cowboys fans. You might as well have told them you teabagged their grandmother because you'll get the same reaction.
There are a lot of fans in the area that root for all 4 major local teams but are true diehards of only one of them. So I will often differentiate between eagles fans and the rest because for example if you tell a Sixers fan you like the Celtics or Lakers or whatever it'll usually be fine. If you tell a Phillies fan you like the Mets or Braves it'll usually be fine. If you tell a diehard Phillies fan who "likes" the eagles that you like the Cowboys in football it'll usually be fine believe it or not. But if you tell a diehard eagles fan who "likes" the Phillies that you are a Cowboys fan, you'd think you told him that you like to torture puppies.