I suspect the answer is that the “large business jets” weren’t actually used for the “sole purpose” of transporting the owner
Some large business jets in private ownership were used to transport gambling folks from overseas to the casino free of charge were they not?
Flew 27 years for a private operator with a large fleet flying company personnel and contractors, had an ops manual stating we would operate to charter standards, never did, nor to CASA standards for that matter, if reference was made to the ops manual the answer was "we're a private operation and don't require an ops manual", operation had its own check and training system. CASA was trying to have an AOC put on the operation, don't know how that went as retired quite some time ago, company had legal talent and political connections that could run rings around anything CASA could come up with.