In the spectrum of things that matter and things that don't, this may not top the list of importance. But we all have the right to know what we're eating, or to think we know what we are eating, and often with wasabi that might not be the case.
"Real" wasabi (below) comes from the stems of Wasabia japonica and is a finicky plant to grow; it's very expensive, grows mostly in Asia, and loses its pungency in 15 minutes after grating. For these reasons, it is not generally found outside of restaurants in Japan.
The green paste we call wasabi? It's primarily European horseradish with the addition of spicy mustard and artificial color. At best. One popular brand I looked up was made of this: Horseradish, lactose (milk), rice bran oil, sorbitol, salt, water, natural flavor, turmeric, xanthan gum, citric acid, artificial color (FD & C Yellow #5, FD & C Blue #1.) Vegans beware! Also anyone wanting to avoid sorbitol or a small rainbow of artificial colors.
This said, there are plenty of restaurants even in Japan that serve the wasabi imposter, which packs a punch and is admittedly palatable, even if it's not what it seems. In the big picture, a wasabi substitute isn't anywhere as egregious as high-fructose corn syrup being passed off as honey or a lowly vegetable oil being disguised as olive oil. Plus there are plenty of things in the Western food repertoire that are merely ersatz versions of something else. But for those who really like to know exactly what they're eating; or those taking caution with what they eat – or anyone with food sensitivities – knowing the details could make a difference.
Here's more on the nitty-gritty of wasabi compliments of Speaking of Chemistry: