When I edit a post why does it remove the sig?

This is because in your profile (as either Admin or Mod) you have switched off your signature:

"Always attach my signature to posts" yes / no

When it's switched off and you edit another users post, if you look at the options underneath the reply box, one of them is:

"Show Signature" with a tick box.

If your signature is switched off, it will have this box unticked, so that when you click on "Update Post" is will also remove the users signature from the post.

You have 2 options to sort this:

1) Turn your signature on in your profile.
2) If you don't turn on your signature, every time you edit another users post, make sure to tick the box that says "Show Signature", before you click on the "Update Post" button.

Hope that helps.

Take care,
Shaun