The reason you never noticed the instruction is that its
fairly new. It was begun with Kodak Sepia II toner. I talked
to Thom Bell at Kodak and he confirmed my guess about it.
Here is the reason: when you bleach the image it is
converted to silver bromide. Silver bromide is sensitive to
light and can be converted to metallic silver by the action
of strong light. This is called photolytic silver and the
effect can be seen on any unprocessed photographic material
left in strong sunlight. The idea of using a safelight is to
prevent the generation of photolytic silver in the bleached
image. However, its really overkill. Normal room lighting
isn't going to generate enough photolytic silver to be
significant. So, in theory, using a safelight is a good
idea, in practice, it really isn't necessary. But, toning in
daylight is probably not a good idea.