A little bit of background for perspective.
The technical and proper term is actually sake lees (sake kasu 酒粕) vinegar. The key ingredient is the lees from sake production, which is left over from the process of pressing the mash//mixture (rice, water, yeast, koji). In essence the sake lees is white in color. In the old days, rice was very expensive, thus rice vinegar was not cheap.
Then came the Edo period, and demand for sake grew. The lees, although a by product, was cheap and readily accessible, and people came up with the idea of using sake lees as a replacement for rice. They added water to the lees to try to make rice vinegar (lees already has some alcohol so it would be less processing than fermenting rice for vinegar), but the resulting flavor turned out to be far too light (not enough sugar content in the lees for further fermentation). Apparently the discovery and breakthrough came by aging the sake lees for at least a year, resulting in a much darker (crimson dark red) shade, then adding water as part of the sake lees vinegar process. Since this was the discovery of its time, hence it was the Edo style vinegar or red vinegar / sake lees vinegar.
Seafood used in Edomae cuisine (including sushi), such as Kasugodai, Kohada, Anago, Kuruma Ebi, Saba, Maguro, Hamaguri, Shako, Unagi (kabayaki, not served as sushi), required advanced prep, involving techniques such as vinegar marination, soy sauce marination, or soy sauce simmer. These resulted in what we would call strong flavored neta/seafood toppings. Supposedly Hanaya Yoshi/Hanaya Yohei, the inventor of Edomae Sushi) decided that rice seasoned with sake lees vinegar was the right fit for the stronger flavored neta. Again, this was also the best available rice based vinegar at the time, as rice (not lees) was expensive and using it to make regular rice vinegar was cost prohibitive.
As time went by, people started to be able to afford and develop tastes for exquisite delicate white fleshed fish (madai, kue, fugu, karei, hirame etc). There are schools of thought out there, that if you pair delicate flavored white fleshed fish, without treating it properly, and serve it with sake lees vinegared rice, then you throw out the entire balance. In Tokyo at the recently minted one star Sushi Arai, actually has this problem where their rice is too vinegary/salty and it is wasted with their white fleshed fish (amongst a few other things), and it was only when heavier flavored neta were applied (e.g. anago with sauce) or pristine uni that was rich, that overpowered the rice to achieve a tilt in the balance towards a bit more of a favorable bite. Some say the stronger salt/vinegar accent in the rice is intentional (even if the balance is thrown) and some locals and fans enjoy and perhaps disagree with that. Different strokes for different folks.
This is also a good reason why Mori Sushi should not use sake lees vinegar in their sushi rice, it’s already perfectly seasoned as is and his seafood selections are so delicate already.
In Tokyo there are some places (particularly the Sushi Sho schools) that offer two kinds of sushi rice, the regular white (white rice vinegar/komezu) and sake lees vinegar seasoned sushi rice (akazu / aka shari), carefully selected to pair with particular seafood/neta/toppings. This is a lot of work, so the quicker way is to figure out an adequately seasoned sake lees vinegared sushi rice with the hope and intention that it works with everything, for places that don’t use white rice vinegar.
While sake lees vinegar is not too sour or strong by itself, it does require further seasoning in order to make good sake lees vinegared sushi rice taste really good (and to match the fish it goes with), and this could be done with sugar (in some cases), salt, konbu and the rest and proportions are industry secrets. Each restaurant/chef will have their own unique blend (using the sake lees vinegar base), some work better than others.
So yeah, many places (from California to Asia) are now using the sake lees vinegar to season their sushi rice, but there are some who don’t know what they are doing. The plethora of places doing this could mean business owners/chefs jumping on trends, thus trying to define sake lees vinegared sushi rice = high end. What distinguishes the great places from the ones jumping on trends is the end result and balance, and of course the folks who understand this and appreciate the ones doing it for the right reasons.