Because if you are not 100% compatible with bourne shell you shouldn't automatically execute script made for bourne shell.
I have no problem with people using #!/bin/dash shebang for their scripts and using dash as their default shell but it shouldn't pass itself as something else. It is fine with bash because it is bourne shell compatible.
> Because if you are not 100% compatible with bourne shell you shouldn't automatically execute script made for bourne shell.
The same can be said about bash.
> It is fine with bash because it is bourne shell compatible.
I was running an HPC cluster when we upgraded Debian during the bash->dash change, and there were all sorts of problems: IIRC most folks simply changed their scripts from /bin/sh to /bin/bash.