What's different this time is that FUSC is (reported to be) limited to the initial buyer of the car, which might be three to five years on average. It serves to differentiate the higher end S and X from the 3 and Y and it seems likely that S/X buyers are more apt to have and use home charging than 3/Y buyers.
So, my take is that the cost to Tesla for this promotion figures to be fairly small. It might become a permanent feature of the S and X, although that's a WAG and the only thing certain about Tesla is constant change! [Watch them pull FUSC in a few months and prove me wrong...]