From a purely philosophical standpoint you are right--as long as we remember that the existence of gray doesn't mean that there is no such thing as black and white.

But I find that moral ambiguity tends to be used as a weapon rather than as an argument.

Most people who raise the "shades of gray" argument to defend their own positions have no compunctions about assigning absolutes to those things that they don't like.

Try and convince a liberal that there is even the most astronomical chance that they may be wrong about the abortion issue, or any of the other hardline positions that many liberals take. You'll see the "black or white" attitude pop up fast enough then. To be fair, the same is obviously true of many positions conservatives take. In both cases, sometimes they're right. There are things that are either good or evil. Not everything, but some.

The point is, they are often consistent in their "no absolutes" philosophical position only as long as it serves the agenda.