Pretty irrelevant "argumentum ad verecundiam", but for the sake of argument, you are not "average" anymore. Being able to pick Schlage and Kwikset already puts you in the probably 0.1%.
Anyhow, back on track, the average customer does not refill their cartridges. I doubt the idea of doing so has even sparked in their head. (and yes, the average consumer is dumb).
What percentage of the general population knows how to dump an I2C EEPROM, do some reverse engineering to find the counter, and update the counter value? It seems easy to us because we're on HN, and many of us are EEs or other embedded systems people, but it's pretty specialized knowledge in the population at large.
This is absolutely DRM. Not effective DRM when squared off against an embedded systems engineer, but DRM nonetheless. Yes, they could have used a hardened encrypted EEPROM specifically built for these kinds of applications (for example, the ATSHA204A from Atmel/Microchip), but that would have cost more and in applications like this COGS for the disposable element is king. They probably assumed that a simple unencrypted I2C EEPROM would be good enough, and quite honestly they're probably right: unless somebody goes and starts selling a little dongle to reset these cartridges to consumers, it's only going to be a handful of technically-inclined power users resetting them.
Yes, and this is why this rudimentary hardware DRM is sufficient for the current market. If the marketplace changes and the hack becomes easily attainable and widely used, SW DRM may emerge.