It was done for pure politics to ensure the need for Shuttle. And it was criminal. Jerry, I read Spinrad's article and he kind of missed the point. NASA could not have used the SDI funds because without the SDI program those funds would not have existed. Betting that the Pentagon would need to help fund NASA to get SDI in space is a pretty good bet.
Some space scientists try to play the same zero sum game in the great manned vs. unmanned debate.
They claim that money spent on the shuttle and space station would be better used for sending unmanned probes to the planets. What they fail to understand is that without the manned program there wouldn't really be an unmanned one. In any case, I don't think SDI took one penny from the regular NASA budget. In fact I think the push for SDI probably saved a few pet projects that would have been dumped otherwise.