Cost to do cable work at LBS?

It isn't and you're right.

Its not as if you brake with one brake to indicate left and the other to indicate right.

Signalling left, or in the case of right sided countries, signalling right. Is something you need to do much less often as you can usually make the manoeuvre without affecting anyone else.

Often getting yourself into the centre of a road/inner lane so you can turn right involves building some speed, which you then have to immediately scrub to turn right, and if you often have to do this one handed too! Personally, I would much rather be using the rear brake when doing this.