A username-and-password login makes sense for a system multiple people access remotely from different places. A shop counter with one device sitting in one physical location is solving a genuinely different problem, and a full login system isn't automatically the right fit for it.
What a full login is actually protecting against
Someone accessing your account from anywhere on the internet, without physical access to your device — remote access is the threat a username/password combination is built to prevent.
What a PIN is actually protecting against
Someone picking up the physical device and opening the app without permission — a coworker, a customer, anyone nearby. This is the more realistic risk for a single-device shop counter, and a PIN solves it directly.
Why a 4-digit PIN is often the right amount of friction
Quick enough to enter dozens of times a day without slowing down service, but still enough of a barrier to stop casual, opportunistic access. A full password with complexity requirements solves a problem this setup doesn't actually have, while adding friction it doesn't need.
When it's worth reconsidering
If the device itself is shared more broadly, or if there's a specific reason to worry about targeted access, a stronger lock (or moving to a system built around proper multi-user login) starts to make more sense than a simple PIN.
The lock is optional for a reason
Not every shop needs even a PIN — a device that never leaves a locked counter, watched constantly, may not need any lock at all. The option exists for the situations where it genuinely helps, not as a default requirement.