Thank you. This is very true. The same holds true for other products in other industries. The new clothes washer we recently purchased came with a sample size box of a particular brand detergent and a dire warning that any other brand could ruin our clothes, ruin our machine, ruin our warranty and so on. It suggested using another brand would summon Sauron from the dark tower.

Several years ago my brother-in-law helped me change the oil and filter in my then-newish Honda Accord. The new filter was a reputable brand, but not Honda. When I soon thereafter had reason to take the car to my dealer, they lifted the hood saw the filter and all but fainted as they erupted with similar dire warnings about the quality of the filter, the damage I'd no doubt already visited upon my engine and the possibility my warranty was in danger.

Honda wants Accord owners to use Honda parts and accessories. Nikon USA wants people to buy Nikon batteries, lenses, flash units etc. Many people here use Tamron, Sigma, Tokina lenses on Nikon bodies with no ill effects at all.

It's called marketing.