The open-source video aggregator Miro actually has a Firefox extension that automatically appends Miro's affiliate code if there wasn't already one on the link you followed to Amazon. This is done pretty obviously so I assume that Amazon is aware of it and allows it.

I don't see anything even slightly fraudulent about saying, "Amazon pays us commission on sales made with our affiliate code attached, please buy expensive things there", and I don't see how that's bad for anyone.