shebanged script not executed right - wth

I have the following setup, which works as intended on multiple quite
different penguin systems:

$ ls -la /opt/compat
total 16
drwxr-xr-x  2 root wheel  512 Aug  8 19:46 .
drwxr-xr-x  4 root wheel  512 Aug  3 13:17 ..
lrwxr-xr-x  1 root wheel   19 Aug  3 15:16 bash -> /usr/local/bin/bash
lrwxr-xr-x  1 root wheel   19 Aug  3 13:18 perl -> /usr/local/bin/perl
-rwxr-xr-x  1 root wheel   45 Aug  3 13:18 python3
-rwxr-xr-x  1 root wheel 1462 Aug  8 19:46 relaymail
$ cat /opt/compat/python3
#! /bin/sh

exec /usr/local/bin/python3 "$@"
$

On the penguin systems, of course it's /usr in place of /usr/local.

The idea is that scripts can put these locations for the interpreters
into their shebangs, so I can use literally the same script on
penguins and daemons.

Like relaymail here - that was supposed to be the first such script.
It is a python3 script:

$ head /opt/compat/relaymail
#! /opt/compat/python3

import sys
from argparse import (ArgumentParser, ArgumentDefaultsHelpFormatter)
import os
import subprocess
import json

CONFIG = '/usr/local/etc/relaymail.json'
BUFSIZE = 4096

Further details of relaymail aren't important.

And now the problem: this works:

$ /opt/compat/python3 /opt/compat/relaymail -h | head -3
usage: relaymail [-h] [-f ADDRESS] [-o OTHER] RCPT [RCPT ...]

simple mail submission agent with few options

but this fails mysteriously:

$ /opt/compat/relaymail -h
/opt/compat/relaymail: line 3: import: command not found
/opt/compat/relaymail: line 4: syntax error near unexpected token `('
/opt/compat/relaymail: line 4: `from argparse import (ArgumentParser, ArgumentDefaultsHelpFormatter)'

well, not completely mysteriously -- it sure seems like it's trying to
run it as a shell script ???

WTH WTH ??? IS THIS A KERNEL BUG ???

-- 
Ian