behavior of stdin with uncompressing

Behaviour of snzip is not difference with other same kind command such as bzip2 or gzip.

    $ cat test.txt.bz2 | bzip2 -d
    foo bar baz

    $ cat test.txt.sz | ./snzip -d  # different behaviour!!!
    I won't write compressed data to a terminal.
    For help, type: 'snzip -h'.

When the command use with stdin, only snzip complain.