Good point - this wouldn't such be a great solution if it caused problems for other people. That has been accounted for, though: the sign actually says "Sonderfahrten" (special trips) in the area where the bus number(s) would be, to indicate that there is no regular scheduled service.

Well, a tourist would probably not know German(as was my case when I visited Austria a couple of weeks ago)

Sure, but one would hope that they -would- know how to look to see whether the number for the bus they wanted was present.