Simply put, it’s because he involved himself in a plot to murder. It doesn’t matter who. It only matters that. Further, he apparently spied for the United States. Theologians have no business either spying or murdering. When Bonhoeffer did those two things he abandoned his office of theologian. And no one who does that can be a real hero or model or mentor for anyone engaged in the theological task.
UPDATE: Be sure to check out Gavin’s thoughts on the topic. To him I would say, true- every Christian should be held to the same standard. I would just point out that theologians, of all people, are responsible, by their own actions, to be examples of ethics. That’s why Bonhoeffer comes in for special scrutiny, like it or not.