That's not to stop potential damage to the drum. It's because on average the drum starts showing damage around 30.000 pages (a damaged drum it leaves lighter or darker bands on the paper, same as on old badly-maintained photocopiers). But because _sometimes_ it starts shows damage earlier, and they don't want you to confuse a damaged drum and an empty toner cartridge, they just force you to change it around 20.000 pages.
The drum reset refill chip is not even a chip, it's a 56 ohm resistor that acts like a fuse. The drum has two resistors in parallel, one with very low resistance and one that's like 200 000 ohm. If the printer sees a high current when you turn on the printer, the firmware thinks it's a new drum and resets the page counter. After a short while the 100 ohm resistor burns (quite literally), and from that point on the firmware will only see a low current when you turn on the printer, and it will not reset the page counter.
So the drum reset refill chip lets you connect the two pads where the printer applies the current with a resistor like the one on a new drum. You can buy a 56 ohm resistor yourself to achieve the same effect.
It's literally a resistor and some plastic, only usable once, and they sell it to you for $10, which makes it much more of a scam than anything printer manufacturers do.
