Once a diver is certified they can dive with who they chose. However until they are certified It's up to me who they dive with.
Its up to THEM who they dive with. Always.
The arrogance of insisting otherwise is simply breathtaking, and is one of the reasons that I find this "business" disgusting.
You've got the entire concept here backwards. There is no scuba "license" (although one could reasonably conclude from your posts here that you'd love to see one mandated and enforced by the government at gunpoint!) You don't need to be certified - it may be smart, but its not required. The student is the one setting the requirements, since its their money. You don't like the conditions, you don't get their money. It is really that simple.
You have the right to refuse to form a contract under a set of conditions set by the prospective student. However, that does not mean "its up to you." Your right to make that decision ends with either acceptance or refusal of their request to train them, which can come with the requirement that someone they trust and want to be with for that experience dive with them during their checkout dives.
For those who I care about, if they request my presence as their buddy in the water the instructor in question will either accept that or another instructor will be selected.
That is their decision, not yours, and you can either respect it or not, at your option - but your "right" of control ends right there.