Tear-off menus are a relic from the old Unix days, where it was common for apps to have tear-off menus. The Gimp even nowadays has the option to tear menus off (if I recall correctly, even the Windows version can do this) - but I think in the Gimp it's disabled by default, but you can turn it on in the settings.
Menus that tear off are confusing to Windows users, though, so you'll most likely want to set the -tearoff=>0 option. You can also set the option globally for all menus by doing this:
$mw->optionAdd ('*tearOff', 'false');
With this you won't need to specify a -tearoff setting for all your menus individually.