It's that this propaganda contained false information on both the source and the details that is problematic. Black propaganda is wrong. We don't need it to win, and when it is exposed it undermines our credibility, which undermines our white propaganda and efforts to contrast our values with the values of our adversaries.
I take a similar stand with politicians. I'd rather vote for an honest person that I disagree with on a majority of issues than a dishonest one that shares my political party. For especially honest politicians I go out of my way to vote for them. Trust in government is paramount. The rest breeds corruption and eventual democratic decline.