Fitbit said they "had" to refuse my warranty on account of having purchased the watch from an "unauthorized re-seller" because of clones, but their software already checks serial numbers for duplicates and rejects clones, so that sounds like an excuse.

Incidentally, my next watch was a Garmin, and while I tried to buy it from the official Garmin store on Amazon I found a 3rd party store on the receipt, so I contacted Garmin and ask them if this would invalidate my warranty. They said it would not. They made good on that promise two weeks ago when a fatigue crack showed up in the plastic frame.

tl;dr Fuck Fitbit, Go Garmin!