There are multiple reasons why one might want to disable sign editing:
Conflicts with other mods (e.g. Better Signs & Frames)
Wanting to use another mod for that feature (e.g. Edit Signs)
Wanting to disallow sign editing on multiplayer servers (e.g. using Flan) or in Adventure mode - normally if you can't break the block, you can't change the sign, but sign editing circumvents that
There are multiple reasons why one might want to disable sign editing: