I think there's a reasonable expectation that product reviewers account for which parts of the experience can be controlled by the seller. You presumably wouldn't leave a negative review on a digital music purchase because your internet access is extremely slow.

Granted, this can be a little fuzzy, since it might not even be clear which parts of the experience are controlled by Amazon, the product manufacturer, and other third parties. Receiving a counterfeit SanDisk SD card from Amazon is almost certainly not the fault of SanDisk, yet a lot of people might leave negative reviews on an Amazon listing if they receive a counterfeit product. That's understandable despite it not exactly being "fair" to SanDisk.