I think the most important thing is the "American Way", which is not necessarily something Superman, who is an international (and intergalactic) hero (in the comics) should represent. However, all-around American reporter Clark Kent can do just that.
On another note, Superman usually doesn't bother with fighting for truth and justice, but to save people's lives and property. The truth and justice aspect is much more pronounced in his work as an investigative reporter. So, yeah, it makes sense to me.
