As I and many have said, we can't see this being the case.
However...... Saying that.
The ONLY reason this could be the case, which none of us think it is.
Is if Apple REALLY wanted to make the Stainless Steel model the one almost everyone was going to get for normal use, and would use the strap issue as a reason to "Up-Sell" you to the stainless one.
The Sports one is only meant for sports use, hence it only fits the rubber straps.
The Stainless Steel model is our normal model, we think most people will want to go for, AND it's compatible with the full range of straps.
Using this as a reason, as I say, to upsell you to buying the more expensive model. That would/could be a reason to do this.
I don't think they are going to do it, but it COULD be a legitimate line of thinking on their part.
If the Stainless model was only say another $100, then this COULD work.
That's my view, unlikely to happen, but there could be the above reasoning behind it.