I dealt with this issue. Since it not a sizing issue, I pushed hard with the Ford support team on another site for reasoning behind this. They went to factory engineers who stated it was because of the traction imbalance caused by the potential of three high-grip tires matched with a minimal-grip spare.
Evidently they have determined that the driving risk of the imbalance outweighs the risk of being stuck on the side of the road and subsequent hassle of tire fix/replacement.
I purchased a spare and now drive with ease of mind. If I had to use the spare, I would drive with extreme caution and get it fixed ASAP.
After wear out of the original tires, I recently put on all-season tires instead of summer-only tires. So this should be less of an issue.
Hope this helps.