Two possibilities.
If DOJ was politicized to go after Democrats more aggresively than republicans (as has been alleged) , then they would have been keen to bring additional charges against the Democrat Berger. The fact that they didn't says it all.
If DOJ wasn't politicized by Bush, then they remain, as they should, non-partisan professional prosecutors. In this scenario, non-partisan professional DOJ prosecutors didn't find any further evidence to support more charges on Berger.
Which means all we're left with is speculation and consipiracy theories by rightwing blogs and superfreak