This is very common on some anodized rims. One factor is whether the rim was anodized after production, or if the extrusion was anodized before being rolled into hoops. Pre anodized suffers this kind of micro-cracking during the forming process. This is far less than ideal for most things, but braking would strip the anodizing anyway so it doesn't matter. However, I'd avoid pre-anodized stock if making disc only rims.

You can also get a similar effect after use because the aluminum flexes, and the anodized surface is more brittle and doesn't. You rims appear to be hard anodized, and that type is especially prone to this kind of surface checking. It's OK and will take eons to become, not OK, but if you're concerned, periodically use a wax/oil treatment applying with a rag against the spinning wheel, being careful to stay away from the brake track.

FWIW- Over the years I've had many rims with similar (non) issues, and have never had a rim structural failure of any kind except direct impact failures (aka crashes).