So what? They're not shareholders, they're donors.
Once money is donated to a Super PAC, it's the PAC's money to do with as they see fit. "PACs may make unlimited expenditures independently of a candidate or political party." Short of the agreement to match funds having extra stipulations (or Lessig putting those wealthy benefactors in charge of the PAC) there is nothing they can do about how the money gets spent.
They could, but this PAC has only one goal, so the money either gets spend towards that goal or not. The donors would have to spend so much that everyone in the PAC organisation switches sides and goes for political ambition instead.
Once money is donated to a Super PAC, it's the PAC's money to do with as they see fit. "PACs may make unlimited expenditures independently of a candidate or political party." Short of the agreement to match funds having extra stipulations (or Lessig putting those wealthy benefactors in charge of the PAC) there is nothing they can do about how the money gets spent.