Be careful about trying to reason from first principles on stuff like this. Poison pill plans are bog-standard settled corporate law. The legal principle is that the board has the right (/duty) to judge the value of the company, and the courts, in most circumstances, don't second-guess it. The board can (and routinely does) decide that offers to acquire the company must be routed through the board.

Plans like this are why Microsoft doesn't own Yahoo, and why Carl Icahn didn't buy Netflix.