Here is an experiment:
Try calling a Pakistani "Indian" and see the reaction.
Boom! 90% of the time, you will get him/her growl at you. You may also get a scratch on the face.
BUT...don't be intimidated. Calm down.
Just ask them "what's wrong?"
They will give you different answers, but trust me, most of them are just "blahs".
You will hear something like: "India and Pakistan are two different countries."
You say, "Acha?! I am really sorry, but you don't have your country's name tag attached to you."
They might say, "We are different people."
To tell you the truth, 50% of the time, I can't tell a difference between a northern Indian and a Pakistani unless they start speaking in Urdu/Hindi (I can tell by the distinction in dialect). So, it is palpable that a non-desi is unable to judge the difference between an Indian and Pakistani.
Then sometimes you get an answer like this, "Pakistanis are mostly Muslims. Indians are mostly Hindus."
That's actually a very good reason. But...wait a minute..this oftentimes come from someone whom you can't really tell if he/she is a Muslim or not. I am not talking about wearing hijaab or having pants above the ankle. I am talking about praying five times or fasting. Besides, how am I gonna know if you are an Indian Muslim or a Pakistani Muslim if your both maternal and paternal grandfathers migrated to Pakistan from India in 1947?
By the way, I went to a "Pakistani" wedding recently in Chicago. It was more like an Indian movie. I couldn't even hear the person sitting next to me because the latest Hindi songs were being played on full volume. I don't endorse music, but it is not like Pakistanis don't have good songs, so why do these so-called Pakistanis need to play Indian songs?
Most Pakistanis are simply confused. They watch Hindi movies, listen to Hindi songs, aunties wear saris, their weddings and celebrations are filled with Hindu customs....they are almost "hindu-wannabes", but they hate being called Indian. Inna lillahi wa inna ilahi raj'ioon...
By the way, if you call me an Indian, you will get a growl too but that's for a different reason. I have saved my answer so that people don't steal it :P