The "inherit" statement is not valid in global configuration files like a distro configuration. Using it there correctly triggers an error. However, when the global configuration file includes some other .inc file and that file has such a statement, no error is printed. The line seems to get ignored silently. Restrictions that apply to the actual usage should also be applied when parsing in include files.
I placed a "inherit bash" at the end of meta/conf/distro/include/default-distrovars.inc which is an incude file included by global configuration. I then see: $ bitbake -p ERROR: ParseError at /media/build1/poky/meta/conf/distro/include/default-distrovars.inc:50: Could not inherit file classes/bash.bbclass which seems correct. How do I reproduce the problem you describe?
We ran into it when adding "inherit" to a distro configuration. But I can't reproduce that with current Poky either. Let's assume that this was fixed somehow in the meantime or that the report was invalid.