It's always been "squeamish," because the standard of proof for treason was deliberately raised to an extremely high level. Note that treason is the one crime that is defined in the Constitution, and that the standard of proof is also defined the Constitution. There is a specific reason for that: the Crown had this habit of charging people with treason for saying things that displeased His Majesty. Your dislike of this aspect of the US Constitution comports well with the views of such distiniguished advocates of judicial reform as Josef Stalin and Saddam Hussein.
That has been the case ever since the McCarthy hearings occurred.
It was the case BEFORE the McCarthy hearings occurred, too.
Our government has also been squeamish about pursuing investigations of foreigners commiting industrial espionage, other espionage and various acts which according to the PLA white paper on "Unrestricted Warfare" as well as numerous older Soviet and newer Russian documents fit in with their strategies of fighting wars against the US and West in the economic and cultural dimensions.
We haven't been squeamish about it. Espionage is an extremely difficult crime to prove in a judicial system where one is guilty until proven innocent. Also, kindly note how well the Soviet Union did in fighting the US in economic and cultural dimensions.
THEY LOST.
How should we defend ourselves against such attacks?
When your opponent insists on shooting himself in the foot, your best defense is to keep passing him more ammunition.
Meanwhile, US corporations are undertaking activities in Communist countries, self declared "formerly Communist countries which actively support anti Western rogue nations and, other anti Western countries.
Uh-huh. They also undertake activities elsewhere.
Is this treason?
No.
Do we need to define some new crime which this ethically wrong set of activity is?
I'm sure you'll think of something such as "thought crime," "counter-revolutionary deviationism," or (most likely) "capitalism."