My first thought when it was announced that only US airlines would be allowed to fly into the US was also "protectionism". I felt insulted that the FAA with its poor security (especially on the apron) would cast aspersions at the competence of the UK.
Then I thought some more. It is unreasonable to expect the goverment of a country that has suffered this appalling horror to act in an entirely rational and logical manner. They had no option but make policy on the hoof. The administrators are only human, and obviously a few mistakes were made. It is understandable. I really do not think that there were any deeper motives. The FAA had a massive security task to undertake in a very short space of time, and the staff could not but be distracted by the national grief and outrage.
In short, the FAA made a bit of a cock-up, but it was understandable, and in the circumstances I don't think that it is appropriate to criticise.
There are far more important issues to discuss.
Life will never be the same again.