Authoritarian countries are often good at maintaining a friendly facade. They understand that the legitimacy of the regime depends on keeping the middle class happy, or at least under control. When they consider someone beneficial to the regime, they will try to treat them well. They are not cartoon villains that treat people badly just because they can. They usually need a reason for that.
I've seen the same differences in the attitudes of Chinese and American immigration authorities. And if my memories of childhood visits to the USSR are correct, the same differences existed between Soviet and American authorities. I believe the main reason is that America is fundamentally suspicious of anything foreign. Authorities are more likely to see threats than benefits in international dealings.
Maybe the US is just good at maintaining a facade that they aren't authoritarian. The US border agents are acting in an objectively authoritarian manner while the Chinese border agents are not and yet somehow you conclude that the Chinese are the real authoritarians. It seems like you are a victim of propaganda.
I don't think the point is "China is not an authoritarian regime", but rather that America is more authoritarian than most Americans think, or are willing to admit.
I've seen the same differences in the attitudes of Chinese and American immigration authorities. And if my memories of childhood visits to the USSR are correct, the same differences existed between Soviet and American authorities. I believe the main reason is that America is fundamentally suspicious of anything foreign. Authorities are more likely to see threats than benefits in international dealings.