My interpretation of the 250mA MAX is simply this: If you want to measure more than 250mA, do not use this socket.What is to be marked on the front panel? If it is OK at 500mA should the marking say 500mA MAX?
This is true if the user knows exactly what current he is going to measure. So a good tip is to always use socket "A" unless you know exactly the magnitude of the current to be measured. And if the user does decide to use the "mA" socket first and the current exceeds the 500mA value for a while. Are you sure that the measuring circuits of this device will withstand such a current if a cheap 600mA fuse is installed?
I, for example, am not at all sure and do not want to check it on my copy. I do not recommend doing such a check to other users.