"I took this to mean "impartial" "
Uh, no. It means that the more al jazerra extends it's audience, the more they have to show they are factual and objective.... not necessarily "impartial"
"However it would be typically American to assume that any channel which doesn't cast a favourable image of America is "irresponsible", as this is not palatable for Americans; they are the best after all."
That's prejudice talking. Assumption and not facts.
"The BBC is unimpartial because it has a little support in the US"
Nice strawman argument. Previous posters have already pointed out the BBC sets out to be impartial in it's charter...but it doesn't always end up that way, it takes constant effort. It has nothing to do with "having little support in the US" directly (the BBC's bias). It's just that the BBC is funded primarily by the UK public, so that is the only audience that can affect them directly in mass.
"but when Al-Jazeera is broadcast in the US it will suddenly reform and become more impartial"
Since Al Jezeera wants to branch out, and the government of Qatar has to help it out, it will gradually become more objective and less sensationalist. That's my "prediction". If you think it's incorrect, argue against it.
"I just can't resist pointing out you're glaringly bad American-English."
Yeah, I wouldn't make a great English teacher, my spelling is a bit reckless. Happy?