In The Kite Runner, why does the visit with the pulmonologist became unsuccessful when Baba asks where he was from?

In chapter 12, Amir and Baba go to visit with the
pulmonologist, Dr. Schneider.  The meeting was going well until Baba wanted to know
where Dr. Schneider was from, then "lost it" when the doctor told him he was from Russia
- Baba threatened he'd break Dr. Schneider's arm if he tried to touch him and refused to
be a patient of a "Roussi".

This is, of course, in response
to Baba's deep-seeded hatred of Russia after they attacked his beloved homeland of
Afghanistan.  In the 1970's, Russia tried to invade Afghanistan, forcing many Afghans,
including Baba and Amir, to flee the country for their own
safety.