Bash isn't a standard shell. The Bourne shell (e.g. /bin/sh) is standard. There are Unices where bash isn't available (by default, or at all), and there's no reason to hardcode bash as the interpreter for a shell script unless you're using bash-specific extensions. (If you have to ask, you probably aren't.)
This is what's called a Linux-ism: "The Linux distro I use does it this way, so it must be the standard, right?" It's very annoying when porting things to BSD or elsewhere.
It depends whether you count standards as those decided by a committee, or those that people actually use. Sure, the various BSDs are quite active (and all have bash available through modern package management systems), but AIX, Solaris and HPUX are on the decline and have been for 10 years now.
Rational ignorance. The bash warnings about non-POSIX features have a reputation for being unreliable, so if I've only tested with bash (because I don't care enough to set up chroot) I shouldn't claim it works with any other shell.
Well ... maybe using a shell to run a command worked differently in 1971. There's $IFS ... job control ...
Okay, yeah, there has never been a shell on which that wouldn't work. WTF bonus points for requring /usr/local/bin, which is almost certainly not where the OS maintainers package it.
This is what's called a Linux-ism: "The Linux distro I use does it this way, so it must be the standard, right?" It's very annoying when porting things to BSD or elsewhere.