I think that’s illegal in California.

But also, that would mean you could assume in a row of cars, the ones with tinted windows were most worth breaking into (which would also defeat the traditional “keep expensive things out of sight” mechanism).

I imagine it's only legal up until a certain visibility is crossed. Usually tint is measured as VLT (visible light transfer), and the lower the percentage the more dark the tint. Most states allow tint down to 30/35% VLT, which is alright but nothing special.

My current car had really dark tint (maybe 5-15% VLT) when I bought it, and didn't really notice because I was interested in the vehicle. After I bought it, I realized that the tint isn't even legal. However, I've had the car for several years now and haven't been pulled over for it, and also haven't been broken into. Truthfully, I think tint is a detractor, because the purp can't see much, so they don't chance a B&E for nothing. I've become a fan of dark tint and I'm planning on putting it on all my vehicles.