It's a "we have no idea when these companies OR our own government might suddenly screw us out of the software we need to sell our hardware, due to changing geopolitical winds" thing. A bit like avoiding your whole startup being only a third-party Facebook or Twitter developer. :)
The US has no problem purchasing foreign telecom gear from foreign companies - Ericsson(Swedish) and Alcatel/Lucent(French), just Huawei. Selective economic protectionism. Free market what?