I looked into it a bit earlier, and was very confused as well, so did not post. What is interesting is that the ORD routing actually books into S and T, while the SFO routing books into W, even though it is $100+ more. It seems like this promotion is actually one which is a good deal, for once.

I am guessing that this is simply a routing issue, and maybe the promo only allows you to connect through SFO. If you are interested in more mileage, though, I did find some better routings, all of which still go through SFO in the end.