I can confidently say that I personally saw a sea otter about 15 miles north of Santa Barbara about 5 months ago that was snacking on purple sea urchins. It would go down, come up, do something, and repeat, and I came across several very-recently-eaten purple urchins washed up along the beach on my walk that morning.

I also pretty regularly collect urchins in that kelp forest, purple and red in that area, and the ones the otter were taking were smaller than the ones I would have chosen. So, at the least, there's at least one sea otter that particularly cares to each some sea urchins, and one human that cares to each the others.

They do, just not emaciated zombie urchins.

It is a bottom tier food source and like you say, they don’t like the evacuated ones present in an urchin barren