Where does the expectation come from that Bash should come ootb with BSD? Or anywhere else outside of mainstream Linux distributions with GNU userland? Bash is the standard Bourne/Korn-like shell for the GNU project, which was envisioned as a FSF Unix clone/replacement. BSD and other Unix-like systems pretty much always had their own separate userland, with often different syntax than the GNU tools.
I think the odd one out was really Apple, which ran Bash as the standard shell for many years (a choice they probably regret), while most the rest of the userland was largely from BSD.
No one says it's an expectation or an obligation. It's a matter of fact: there's no bash and it is obviously inconvenient for people used to bash, that's it.
I think the odd one out was really Apple, which ran Bash as the standard shell for many years (a choice they probably regret), while most the rest of the userland was largely from BSD.