I don't think this is true. If some government agent tells the pilot you are a "security risk" for any reason you are not going on that plane or if you are already in it they will drag you out.
The plane is a private property so you can be taken of for any reason.
Respectfully, you are mistaken. Private airlines are "common carriers" and are required to play nice with the public, and you're conflating "whether the private carrier wouldn't want to take you" with "whether US CBP wants to see the contents of your laptop."
I actually filed suit over the former back in 2015. AA told me they wouldn't let me fly home unless I answered their "security questions." The case was resolved quickly when everyone (AA and DHS) did an about-face and said whoever told me I wouldn't be allowed to fly was mistaken.
The US may not refuse entry to its own citizens, even if they insist on remaining silent. They can detain you to be searched on arrival, but they must, after completing that search, admit you to the country.
For international travel the US doesn't have any say over who's flying to them from some other country. Though if you're a known terrorist or something be prepared to be arrested once you land. And once they've landed you can be dragged off the plane, but if you're a US citizen then cannot deport you. They can take you to prison, but not to another country.