Increased current draw in data transmission circuit
It's a question of switching threshold.

For simplicity, let's suppose that the supply voltage is 10 V, so that the added 10 k&Omega; resistor is drawing 1 mA.

The device receiving the data (Rx) is monitoring the current being drawn by the transmitting device (Tx) from the 10 V supply. If this current is above a certain value, say 5 mA, then the Rx tells the computer that the value is "1", and if the current is below that value, the Rx tells the computer that the value is "0".

So, we'll postulate that the older Tx devices drew either 4.5 mA to send a "0" or 5.5 mA to send a "1".

The newer Tx devices, for some reason, are only drawing 3.5 mA for "0" and 4.5 for "1". Since both of these values are less than 5 mA, the Rx device always reports this as "0". But by adding the 1 mA drawn by the resistor, the total current seen by Rx is now 4.5 mA and 5.5 mA, respectively, allowing it to decode the data as intended.