Why do some back course operations use normal sensing?
Notice that in the plan view, the shading of the localizers are on the opposite sides. If you are flying south-west, the actual localizer used for the approach has the shading on the right, while the one used for the missed approach has the shading on the left. This can also be used to tell you what sensing to expect.  The note is also there to make it more obvious that the sensing will be backwards from what you might expect when flying outbound on a "normal"  localizer.  

So yes, this particular localizer is designed to transmit backwards and you will get indications that would be normal while flying inbound when flying outbound on the missed.

I'm guessing that because of the high terrain/mountains in the area that they wanted to make it as easy as possible to fly without getting confused as to which way you are supposed to turn.  Combined with the fact that it is never used in a situation where you would be flying inbound makes it a perfect candidate to reverse the sensing.  Unfortunately, in my mind it actually makes it harder to understand.  We are taught from the beginning of our instrument training how to handle flying a localizer outbound, and now we have to do it the opposite way.  Rather confusing if you ask me, but you get used to it.

As far as "how common is it", this is the only one that I've seen (but I'm guessing that there are others)....