The word "dangerous" is not exclusively used to indicate the likelihood of physical injury. It is also widely used to describe the likelihood of any undesirable consequences. The undesirable consequence in this case is a weakening of the argument being presented.

How does it weaken the argument? No one has put forth the idea that it is ok to talk to good cops. Don't talk to cops good, bad, or indifferent in nature.

The very blog post which prompted this discussion explains the danger of assuming even impartial investigative intentions of cops, let alone "good" ones.