[refurb]

There are usually shareholder agreements in place that not every single shareholder has to agree to a acquisition. Otherwise, you'd never be able to acquire a company if just one shareholder, holding one unit of stock said "no".

[ptaipale]

I think in most jurisdictions there is a law that enables a supermajority shareholder to take possession by a forced buy-out of remaining stock, at a "fair price". Thus a separate shareholder agreement is not necessarily required.