[aptwebapps]

The optimum point might lie outside. The curve might even dip in between the two price points.

Edit: The operative word here is 'might'.

[nl]

Yes, it might.

But presumably the authors wants to maximize revenue while conducting experiments. Pricing at the midway point would seem to be the lowest risk way of getting additional data.

[carbocation]

True, but wouldn't you agree that nl's suggestion is the maximum likelihood guess?