Actually I don't see any reason why they wouldn't be called pyro's .. even if in slang. The example I throw at your feet is the sherman tank during this very same war. The british called them Ronsons, the americans called them Zippo's, because they used gasoline engines and if hit with a shell had a good chance of exploding from the fuel tanks being ruptured. they were given those names because they were brands of lighters.
diesel engines didn't pose much of a risk of exploding in flames so the german and the soviet tanks weren't as likely to burst into flames.
I doubt this has anything to do with why they were called pyro's but one can dream can't he
History lesson aside .. I assume it's because that's the class of ship, why would we call a Black Swan anything else?? I'm not about calling them "Dear god ALMIGHTY" are you? Black swan works just fine for me.
Wamphyri