It was US pressure obviously, for which the countries apologized. It was an asshole move by the US but this is on another level, it involves foreign civilians, KGB agents, fake bomb threats and fighter jets

Mental gymnastic you are doing there. Spain and France denied the airspace, meaning the flight couldn't gone through either of them. There is no "not to allow the plane to land". In fact an airplane calling emergency has priority to land anywhere, regardless of the political difference with the host country.

On this topic, this happened with a flight from a US's air base in Afghanistan, rerouted to Iran because of a "bureaucratic issue":

[0] - https://www.reuters.com/article/usa-iran-aircraft/u-s-says-p...

No, they certainly didn't have to land in Austria. But somehow, god knows why, certain countries closed their airspace for that specific plane. I'm inclined to belive that more countries would suddenly close their airspace too if plane chose not to land in Austria but to go to some country where Snowden won't be in danger of being arrested - France, Spain, Portugal and Italy are hardly the only one that would bow under US pressure.