I'm not arguing for or against Iran sanctions. However, they exist, and the US has a history of enforcing such sanctions. Getting around sanctions is typically given large punishments (see the Movie 'War Dogs'). This is not some arrest on trumped up charges for 'national security' like the Chinese arrest

The fact of the matter is that there is strong evidence that Huawei committed fraud to breach US sanctions. This is neither a 'good' or 'bad' thing, it is just very illegal.