I don't want other users inserting any kind of footer or sig line in their entries automatically. If they do, they should at least have the trouble of setting up an external client to do it.

So you want to prevent other people from being able to easily do things you personally dislike, out of personal preference?

Given that, as you've said, it can be easily hidden in style, what's wrong with having people be able to write what they want, in the same way we can for the entry that hits the other site from here? I use DW as a client for LJ, that allows me a custom footer on every LJ post that I can edit from here.

Why does DW let me do something on LJ but not let me do it here? What's that substantive difference?