Which shareholders you are talking about?
If you mean the shareholder option as "Alibi" for few weeks now. In real they should own about 99.9% of their hashpower themselves.
Its like they don´t want to be significant more then 25% of the total hashpower because this will noticeable and the community might think about how to handle this, but I am pretty sure they have the ressource for having even a lot more then 25% hashpower.

At least its possible to handle this kind of abusing with a kind of "network limit for hashing"
This way you could prevent big hashing networks and prevent the whole community even for bigger dangers like the NSA with their own hashing datacenter.