1. Avoid misinterpretation and overreaction. The power relationship is inherently, grossly unbalanced and holos may feel compelled to change their behavior based off implications in private conversation, intended or not.
2. If the big boss has an open line, some people are going to use it, and it places indirect pressure on others to at least consider using it as well. Even holos who think it's better to leave him alone will feel the obligation to. If nearly every other holo was chatting regularly with him and you weren't, you might be seen as one of the exceptions, cold and unfriendly, insular, etc. Same way you know, you don't HAVE to accompany your boss to drinking parties after work, but...
3. Too many holos. There are over 50 talents. Imagine having to keep tabs on that many "friendly" conversations in addition to all your other work.
4. Holos may go over their managers' heads and air their grievances directly to Yagoo. This puts the burden of handling random issues, small or otherwise, directly on him - exactly what the whole management structure is supposed to avoid. The holos may not even do this deliberately, but in the course of shooting the breeze. This makes him have to constantly watch his words to avoid accidentally promising something that he does not have the authority or ability to deliver, because the company has already arrogated it to someone else. Even if he did have the power to overrule that 'someone else', the act of doing so diminishes that person's authority and makes them less effective in the long run. Banning it makes this impossible.
5. Yagoo constantly has to watch his words to avoid giving any hint of impending changes. And acting mum about important things can damage trust later. Imagine something like the board internally discussing raising the cut they were taking from holos. It would feel like getting stabbed in the back if you were shooting the breeze with him one day and then that happened the next.
6. Avoidance of scandal. Can't have any weird stuff going on if you don't talk.
There are probably more, but chain of command even in companies exists for a reason.