Is this common for ecoboosts? why even give the 87 option?
This is absolutely nothing new. Factory cars need to be able to run on all available fuel grades. All the way back in 2006, I had a GTI with the 2.0t direct injection engine and it would run on both 87 and 93---though higher octane fuels were recommended.
What you see with the lower octane fuels is the timing being pulled and oftentimes (at least I saw it with the GTI) a concurrent reduction in fuel economy. The car ran best with at least 91 octane. The 2.3t is likely very similar in that respect.
The other issue is that many people aren't very good at math and think that spending more on premium fuel is too much to ask. A lot of people will not buy a car that only takes premium. I would guess that the acceptance of paying for premium fuel is inversely proportional to the cost of the car. With the mustang being affordable, acceptance of premium is probably on the lower end of the scale.
Personally, I don't think you'll feel much, if any, drop in power between 91 and 93 octane.