THe absolute maximum ratings of a part are not necessarily what it will naturally draw, but what it will tolerate.
THese are rail clamp diodes (ignore the one between Vdd and GND, that's not it). If the line voltage rises above Vdd+Vf, the diode becomes forward biased and turns on draining current in order to clamp the voltage. It is a similar thing for if the line voltage falls below Vss-Vf.
**broken link removed**
THe specific type of diode is used is dependent on the type of overvoltage you want to protect against (ESD, inductive spike, bad voltage source, etc).
IN all acutuality, for these inputs it will probably take a voltage higher than the regular operating voltage to cause 10mA to flow. THere are probably rail clamp diodes with these pins that clamp the voltage to Vdd+Vf and Vss-Vf (where Vf is the forward voltage of the diode) during over/under voltage conditions. As the voltage gets higher, more current flows as the diodes work harder to clamp the voltage, and these diodes can only handle 10mA. There is also likely an internal resistance in series with the pins for the excess voltage to be dropped across (since diodes only have a constant voltage drop so in theory an infinite amount of current would have to flow through them to clamp a voltage). However, for practical reasons these resistances probably aren't outrageously large.
So if you were in a situtation where you knew these diodes existed (or in some cases added them yourself because you need them or needed them to handle higher power) you would add a resistor to limit the current (or further limit the current if an internal series resistance was already there) and give the diodes an additional resistor to drop the excess voltage across in order to allow the diodes to be able to clamp higher voltages.
But this probably isn't the case here since you are doing digital to digital. (BTW, this is how "5V-input tolerant" pins on 3.3V microcontrollers work.