Inkscape never inserts viewBox in its files, so if not you, some other program must have done it :)
Ohh... yeah, I needed to rummage my memory quite a bit, but in fact it was me who inserted that line eons ago when I had the ambitious plan of embedding the real SVG into a webpage.
well, perhaps eons ago this was ambitious, but nowadays it's perfectly fine. 3 of the 4 main browsers display SVGs fine, not all features maybe, but certainly good enough to cover >90% of the static vector file uses. So, maybe give it another try.
I decided to stick with a (Makefile-generated) PNG and forgot all about the viewbox.
I really hope that Inkscape learns to deal with viewBoxes, and nested SVG elements. Both are very important concepts of SVG and are commonly used in graphics and applications on the web.
Andreas