Perhaps you've missed the other epithets that are in common use here on FR, like "muzzi", "raghead", etc.
Here's the explanation: "paki" is not a racial term. It is a cultural term. The Muslims of Pakistan are essentially the same race as the Hindus of India. The difference is that a Muslim (of Pakistan or wherever) is, by the precepts of his religion, compelled to be at war with the "infidels".
I shall not feel myself compelled to consider the feelings of an avowed enemy.
Paki in the US may be a cultural term, in the UK its a racial term.