And we open html-files by default in the html-viewer, because we also use it to show results of some tools, like CCCC or the html build log (if enabled) or some helpfiles or ...
This does not work, if the internal editor opens these files.
So we had to decide what we want.
And as C::B is mainly a C/C++ IDE we did it this way.
But we give you the freedom to tweak it, so it fits your needs.