These two things are not mutually exclusive. The decision to allow operating without a battery is a mainboard design decision and has very little to do with the protection circuitry on the battery pack. In a device like this, the decision to require the battery as the primary current source is usually due to the high current draw for activities like initial searching for a cell tower. Lithium batteries can source this current easily, but if you want to run without the battery, your charger needs to be able to source that max current as well. Sizing the charger to handle this brief load translates directly to additional product cost, so it is often not done that way.

The protection circuitry on the battery pack is designed to protect the cell from charging and discharging situations that may damage the battery. You definitely want those protections in there.

There is virtually no batteries without ICs, and, that has nothing to do with whether the device runs without a battery. That’s just a design choice.

>There is virtually no batteries without ICs

There are after market batteries w/o battery management ICs like TI Gas Gauge, its up to device on whether it will accept a battery without such IC.

>that has nothing to do with whether the device runs without a battery. That’s just a design choice

I agree that just having a battery without IC doesn't guarantee that the device would function directly with an external voltage regulated power supply, but a device which accepts only the battery with specific IC makes it difficult or even impossible to run the device that way.