It is the _directors_ making decisions here, not the shareholders. Those directors were chosen by management, and -- newsflash! -- they are beholden to management. They must jump through some legal hoops to discharge their fiduciary duties and protect themselves from lawsuits, but at the end of the day they'll do what management wants.
Does management want to maximize the value of Twitter? Er, no. Management wants to maximize some combination of their compensation, their agency controlling a company, and their social capital controlling a social media platform. Maximization of shareholders' value is a legal objective to which management must pay legal lip service, but it is not in any way shape or form the interest of management, save insofar as management compensation is related to share price.
If management cared about shareholder value, it would not have induced the board to enact this poison bill nonsense.