A Busführer I would understand as a bus guide (eg: tourguide)
Which ever one is correct depends on the context...
and some Swiss would use the same word for both
but I may also be wildly wrong (I have been in the past, therefore am likely to be in the futre...)
There are many many many words used in the media and in print in the german speaking part of Switzerland that would not be found in the High German Dictionary. That does not mean that in Switzerland it is not a 'proper' word.
And I'm sorry to insist but you don't live in a country where high german is the culturally accepted official language.
A Busfahrer it the one driving the bus (including myself sometimes after night out in town ;-)
Error by ZVV? Perhaps.
Regardless, using the argument that a word is not found in the High German Dictionary and hence might not be a real word in the german speaking part of Switzerland most likely isn't going to be valid.
And bus driver and bus leader could be two different things.
EDIT: Anyhoo, here's the link to an article from TagesAnzeiger where they use busführer.
http://sc.tagesanzeiger.ch/dyn/news/zuerich/825713.html
"Zum ersten Mal gehen die Verkehrsbetriebe Zürich gezielt im Ausland auf Personalsuche. In Berlin sollen zehn bis zwölf Busführer angeworben werden."
On a quick check I guess some are simple misspellings, some where as guessed before little books that guide you on bustravelling and some seem to use the word for a travel guide that leads the group that travels by bus. I, as a German, never heard the term before.
But if the Tagesanzeiger uses it, I guess it is an accepted term for bus driver in Zurich: http://sc.tagesanzeiger.ch/dyn/news/zuerich/825713.html
Ironically the article tells that the VBZ are recruiting "Busführer" in Germany
You have to say Tramführer and Zugführer but Busfahrer. Maybe you have to use führer if there are several vehicles attached in a row and there is a leading one in the front as it is the case in a tram or train.
So to use the word Busführer could just indicate that ZVV has not only buses but also trains and trams. They could also have said Buskapitäne (or Buspiloten if ZVV had planes, too ;-))