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Support for python -m<runnable-stdlib-module> [-h|--help] is a bit patchy right now:
$ python -mpdb -h
usage: pdb.py [-c command] ... pyfile [arg] ...
<help elided>
$ python -mpdb --help
Traceback (most recent call last):
 File "/usr/lib/python3.4/runpy.py", line 170, in _run_module_as_main
 "__main__", mod_spec)
 File "/usr/lib/python3.4/runpy.py", line 85, in _run_code
 exec(code, run_globals)
 File "/usr/lib/python3.4/pdb.py", line 1685, in <module>
 pdb.main()
 File "/usr/lib/python3.4/pdb.py", line 1629, in main
 opts, args = getopt.getopt(sys.argv[1:], 'hc:', ['--help', '--command='])
 File "/usr/lib/python3.4/getopt.py", line 93, in getopt
 opts, args = do_longs(opts, args[0][2:], longopts, args[1:])
 File "/usr/lib/python3.4/getopt.py", line 157, in do_longs
 has_arg, opt = long_has_args(opt, longopts)
 File "/usr/lib/python3.4/getopt.py", line 174, in long_has_args
 raise GetoptError(_('option --%s not recognized') % opt, opt)
getopt.GetoptError: option --help not recognized
# <-- not a getopt specialist but --help is actually listed in the call to getopt!
$ python -mtrace -h
/usr/lib/python3.4/trace.py: option -h not recognized
Try `/usr/lib/python3.4/trace.py --help' for more information
$ python -mtrace --help
Usage: /usr/lib/python3.4/trace.py [OPTIONS] <file> [ARGS]
<help elided>

Please open individual issues to address individual modules that you would like to contribute to improving. Adding/fixing help is often best done by rewriting the argument parsing...contributions have been made to improve several modules already. In most cases we don't have tests for the -m features, and those need to be created first. In some cases we don't really want to continue to support whatever -m code exists (because it exists for no-longer-relevant historical reasons...though that doesn't apply in the two cases you reference). So each module needs to be addressed individually.
To address the specific issue you raise: I believe that when we have rewritten things, we have chosen to follow argparse's lead and support both -h and --help for the display of help information.

To be honest I don't really plan to contribute any patch on this specific issue right now, the CLI interface of e.g. trace has some other serious issues (see e.g. #24649) that I don't want to work out, and writing tests for the CLI are a pain too (also why I haven't made more progress on #23596).

Yes, that's pretty much why things are in the state they are in ;)
Still, opening individual issues for help problems with individual modules is the way to go, as you did (thank you).

> writing tests for the CLI are a pain too
It shouldn't be particularly difficult to do it using script_helper.assert_python_{ok|failure}(), even though you could also check the argument /parsing/ separately without having to launch a subprocess.