The allegation is not quite that Huawei imported American goods into Iran. The accusation is that Huawei misled a non-American bank, HSBC, about its relationship with Skycom, a company that is accused of importing some American office computers into Iran. The US alleges that Huawei essentially controls Skycom, and that it hid this from HSBC, a British bank. Since HSBC does business in the US, it would not be allowed to give loans to Huawei.
You can start to see how far-reaching American sanctions are. A British bank is being told that it cannot do business with a Chinese company, because that Chinese company allegedly controls a Hong Kong company that imported small numbers of American goods into Iran. As I understand it, these sorts of extensive secondary sanctions are not normal, and are a sore point between the US and many countries, including some of its allies (such as France and Germany).
I would be very interested to know what gives the US any jurisdiction to demand the arrest of a foreign executive on foreign territory, for acts committed on foreign territory. Whatever the legal justification, though, this is a very unusual and aggressive step, and one that is transparently motivated by political considerations in the ongoing trade war.