Think the plan for S is probably a bit different.
Maybe it's just me, but kind of see S as becoming the default install for OEMs. Why pay for an OS when they can use a free one from MS? Makes sense for them to do so as if the customer wants more they can pay the Pro upgrade to MS.
Why might MS think this is a good idea? Mostly as it's direct cash to them for the upgrade fee, but it is also a huge push for developers to make for the MS Store as it could well become the default for all new machines. Maybe it's just me thinking this could be the move, but nothing else really makes a lot of sense.