They banned importing internet-connected foreign-produced inverters. (https://docs.fcc.gov/public/attachments/DOC-423682A1.txt) They are perfectly legal to sell without built-in connectivity, and external monitoring methods aren't prohibited. The concern is that a coordinated network attack on solar inverters could bring down the power grid.

They don't go into details, but check out the Aurora Generator Test: https://en.wikipedia.org/wiki/Aurora_Generator_Test Running solar inverters in opposite phase to the grid would likely not immediately damage them, instead they would draw the amount of power they'd normally provide. In some parts of the US, especially during peak summer demand, even a small percentage of solar power doing this could cause a cascading outage, knocking entire multi-state power grids offline. (See also: https://en.wikipedia.org/wiki/Northeast_blackout_of_2003)

Granted, this could still happen with compromised US-produced inverters, but it's something we should be paying close attention to and planning around.