You're under warranty, so I'd have them replace both. If you weren't, or if it were a tremendous inconvenience, I'd fix it myself (if possible).
I had that happen to one of my Tiger's "sealed" fog lights. I took it apart, dried it out, and sealed it properly. You might be able to do the same for the turn signal.
The headlight, like someone else pointed out, should have a port (weep hole) that allows moisture and moist air to exit, but if there's a leak then it's possible for water ingress to overwhelm it. It's also a sealed unit, but it might be possible to disassemble and seal it correctly as well. If not, you can try putting it through some hot/cool cycles to force the moisture out, then jerry-rig some backyard solution like sealing tape around the seam where the lens is mated to the unit's housing (not the external housing or bezel, which aren't part of the unit.)