It could be IoT. One of the biggest challenges of IoT is connectivity. It's pretty hard to build reliable IoT implementations with 99% uptime on top of WiFi or BLE (and even harder when relying on broadband cellular networks).

There are many critical systems that in order to take advantage of connected technologies require dedicated or special protocols that can provide speed, reliability, high security, low latency, low power consumption, low data rates.

Most unlicensed radio gear chooses between the 2.4GHz and 5GHz ISM bands. 2.4GHz offers longer range and better penetration of walls, but a lower available bandwidth (fewer/slower channels). 5GHz offers better bandwidth but lower range and less penetration. Perhaps 3.5GHz provides a happy middle ground for Amazon? Not to mention there is much less interference on that band because they don't have to put up with everyone's wifi, bluetooth, microwave ovens, etc etc.

Generally you use whatever spectrum you can get rather than having the luxury to strike beard and decide which spectrum you would prefer.

When available for non-military use, 3.5GHz will be attractive because it will be empty of encumbant users. Therefore far more usable than 2.4 and 5GHz, provided the cost is reasonable. Also likely much cheaper than the other available P2MP bands (principally allocated to LTE in the US).

"strike beard"?

Maybe "Stroke beard" as in " rather than having the luxury to stroke your beard and decide which spectrum you would prefer.".

I'm imagining like a cartoon evil villain stroking their beard/chin.

If it's free for everyone to use (on the "lowest" tier) like 2.4 and 5GHz, won't it be quickly saturated as well?

Hopefully it'll be sliced into smaller channels than 2.4 was. Also, 3 frequencies gives more room than 2.

In their application letter, they clearly say they are using it for LTE (specifically on 10 and 20 MHz channels): https://apps.fcc.gov/els/GetAtt.html?id=219961&x=.