Of course it's expected that many cyberattacks can originate from the US, as there are many well-connected zombie devices here. Also, the US also has approximately 10 times as many datacenters as the next country in the list, so it's likely a malicious actor would leverage a cloud service in the US for many attacks, for example, and it's an easy way to mask the true origin of the attack.
With regard to purchasing Chinese-made devices, there is perhaps some ignorance in this area. It's unlikely that the CCP would force manufacturers to add "backdoors" to exported products wholesale under normal circumstances, since if discovered it would cause sales of Chinese products to crash. Also, backdoors get a lot of attention, but in reality there isn't really a difference between a backdoor and an exploit that only you know. So modern "backdoors" are probably just snafus that were discovered but kept secret in case it might be useful in the future, especially for high-value targets.
The primary concerns typically revolve around two areas, in my opinion.
First, a lot of Chinese products are popular due to their low price, and it costs money to hire skilled coders with security experience, provide continued firmware updates after the sale and offer bug bounties to keep your product secure. So ultimately a lot of the cheap stuff we buy ends up being a security risk not specifically via CCP pressure, but simply due to the targeted price point.
Second, there IS an elevated general risk of devices being exploited in the future, especially if there was a conflict. Plenty of devices phone home and/or check for/download updates automatically, and this would absolutely be a major attack vector.
So ultimately it's very logical to simply ban such devices out of caution, and that is also simpler than having to test and maintain an exhaustive list of approved/not approved products.