Pace
It is not just about being safe. It is about being seen to be safe, with a paperwork trail to prove it and accountability. If an AOC operation is not safe then the CAA is likely to notice and the Chief Pilot, Chief Training Captain, Ops Manager, CAME Manager and ultimately the Accountable Manager are responsible for that, answerable in a court of law. They have been interviewed to establish their suitability and sign a piece of paper to accept liability.
In an illegal charter there is no such accountability. I believe in a corporate charter there is a certain level of accountability, under an employer's duty of care, although the structure is not defined. Many corporate operations are modelled on AOCs, and of course they will be as safe as long as they follow the same rules (such as FTLs, and crew training). This is not alays the case though, and there is no way of knowing from the outside if it is. That is why they can't charter the aircraft.