Where did I say preferential treatment is bad?
I think you are missing my point.
Here's another quick take:
If I am going to enter into a non-trivial business relationship with a supplier and I have multiple equivalent options, I will avoid a supplier with potential conflicts of interest with competitors. If it is important enough for that supplier to get business from many potential large customers, they must avoid having conflict-of-interest issues.
I have a relevant example from a project we are working on right now. It's a specialized aircraft simulator.
We have about $250K in CNC laser tube cutting and welding work we need to outsource to a vendor with the right capabilities. We have been in conversations with about a dozen of them. One of them has a consulting relationship with a company that is a competitor to our client. Not a direct competitor (not the same product) but close enough to raise a question. We had to disqualify this vendor on that basis.
In fact, this sort of thing is so important in business that the vendor actually brought up the conflict of interest to us. We didn't know about it. They informed us that a potential conflict existed and suggested they should not be be considered for the contract.
That's what I think Panasonic is doing. If they want to go after relationships with direct Tesla competitors it is a lot easier to manage if there are no conflicts. Owning billions in stock on one company and none of the others is a clear conflict.
Remember that owning stock in a company means you are an owner of the company. It is YOUR company. Becoming a supplier to companies that compete with a company you own is a very clear conflict minefield.