The market value of the non-voting shares should theoretically be the value of the voting shares minus what the market believes the value of voting to be. If a stock buyback only buys voting shares and the price of shares in that class rises, such that the difference in prices of the share classes is greater than the value of voting rights, then an arbitrage opportunity exists. Thus the market will bring the price of the non-voting shares up, and the shareholders of that class will benefit too.