At first I thought you were talking about outputs. What you describe is pretty common for outputs. Solid state discrete PLC inputs usually have a pretty high impedance and may "consume" a very small amount of current when activated.
It is a common practice have a fuse or breaker protecting the wiring and devices which may then be wired to the inputs, but it is not really necessary to do that to protect the inputs themselves.
If you have many circuits spread over a wide area that ultimately end up in the PLC panel, it can be useful to group them by location and protect each group. It can be useful to have a separate circuit for things that are of critical importance to a process so that if some other less important circuit fails and trips its protection, the PLC is still able to gather information from the things that are deemed important.
Usually when PLC inputs (talking solid state discrete inputs) fail, it is because there was some sort of a voltage problem...ie incoming spike.
Analog inputs are a different animal. a 4-20mA is often protected by a 0.063A fuse. Why? Probably because that is a readily available component that is not too close the the normal operating range of the circuit. You can find 20mA fuses but then they'd blow under normal conditions. I have seen 1a fuses used and fail in a fault condition. I have seen a transmitter fail and try to pass 120mA to an Allen Bradley analog input without any detectable harm. The data was maxed out, the sensor replaced, and all was good to go. In series with the input, I measured 44mA. Connected to my meter, it was 120mA. That case did not have individual protection on the inputs. I probably got lucky that day that the input was not damaged.
Tell us what type of PLC you are dealing with in this case...maybe my advice is off-base.
