Yeah, this used to be a problem on 'buntu, and it only got fixed when they switched /bin/sh from Bash to Dash.
People who write scripts tend to be careless about such things, since everyone is so used to Bash. While I consider this to be inexcusable, it is also very difficult to enforce.
Raising issues for every single package is futile – it may fix some instances of such scripting malpractice, but it won’t solve the general problem. A large majority (almost all) of these comes from upstream, and is overlooked by Arch package maintainers.
Without a firm, uncompromising stance saying: “we won’t accept any script into Arch repos with #!/bin/sh if it is not strictly POSIX-compliant”, this will never get fixed.