Why do dark colored items have care instructions that say to wash with light clothing?

Honestly, you should be just fine to wash this with other dark colored items. There are only two peices of advise I would offer:

(1) If you wash this towel with dark clothes, it might leave dark blue fuzzies on them depending on how "plush" the towel is.

(2) If you wash with anything other than other dark colored items, I would suggest washing it with other towels. It could very well bleed onto other items like socks, shirts, etc.

Some items have a sort of generic label that the company can sew on regardless of whether the item is light or dark. In other words, it might have instructions for both lights and darks, and they just assume that you will figure out whether it's light or dark. Obviously a dark blue towel will not show anything if other darks happen to bleed onto it, so I'd say your instinct to wash it with the darks is correct.

It may be a mistag or something. I have never seen something that didn't say "wash with like clothing."

Come on....use your common sense.