Probably the part where keeping everything local would allow the driver to easily bypass the device. Splice a few wires, and boom. But if it is doing some off-site verification, they’ll be able to immediately know if the device is disabled. Similarly, they could do things like monitor the car’s location in real time, and have it throw up a red flag if the car is moving but the driver hasn’t performed a test. That would be a sign of tampering.

It also allows them to know if the driver fails the test, which is important for probation/parole reasons, where not drinking is often a condition of release. So if they fail the test, it should automatically alert their supervising officer. Can’t do that if it’s all local.