Here's the rationale given for ignoring SHELL in POSIX system():

    One reviewer suggested that an implementation of system() might
    want to use an environment variable such as SHELL to determine
    which command interpreter to use. The supposed implementation
    would use the default command interpreter if the one specified
    by the environment variable was not available. This would allow
    a user, when using an application that prompts for command lines
    to be processed using system(), to specify a different command
    interpreter. Such an implementation is discouraged. If the
    alternate command interpreter did not follow the command line
    syntax specified in the Shell and Utilities volume of
    POSIX.1-2008, then changing SHELL would render system()
    non-conforming. This would affect applications that expected the
    specified behavior from system(), and since the Shell and
    Utilities volume of POSIX.1-2008 does not mention that SHELL
    affects system(), the application would not know that it needed
    to unset SHELL.