They frame this as fraud-prevention but I don't really buy it.

I think of I this way. As a casual/hobbyist user, maybe I'm happy to accept a 10% risk of the device failing prematurely in exchange for saving $20 on a memory upgrade. Ok, so these unscrupulous sellers will never tell me that info up-front...but this defective-by-design system doesn't help me either. The sellers playing this shady game might not accept returns (one example of this already in the github issue linked from the main thread) and there's no sign anything's wrong until you receive and boot up. So the user is left holding the bag, with kneecapped memory capacity, and the same stability risks anyway!

If these DRAM-upgraders are really so clever, they'll find a way to bypass this (patched firmware or hacked serial in the CPU). At least, it'll work until the user is out of the 30-day period when they finally update the firmware.

Or if the CPU/mainboard can really detect this in a non-defeatable manner, why cut the usable memory? Just print something to dmesg or stderr when the user logs in. "This device's memory has been modified and may be unstable, warranty blah blah"

Apple locks down fingerprint/camera module swaps, but at least there's a plausible (afaik) security rationale for it. Seems like Raspberry just got tired of being undercut on price.