Hugh,
You will find that nearly all forums have a set of rules. Those that don't are usually a den of chaos. The line has to be drawn somewhere, and if someone has clearly crossed that line then there is no argument as to why their post was deleted. The rules are not onerous by any means and they make it clear and simple for all concerned where they stand. If inflammatory, hostile or insulting posts are left as you suggest, then it is a clear invitation for anyone to retaliate in a like manner and that always ends in an ugly flame war as we have all seen before. It not only ugly but also damaging to the forum, with new and existing members unlikely to stay as a result.
If you wish to discuss this further feel free to pm me, but it is not really matter for discussing in public on the boards.