That's really interesting, but does make sense. I wonder if you could quickly photoshop out the badges, as I assume you had the badges on the car too. This is why GTAV name their cars differantly, the SHAPE and design of the cars are more or less the same, but they changed the name and logo on the cars and it seems that's how they got past copyright issues. So in that regard my guess would be on the branding and naming more so than the design of the car it'self. I see lots of artists doing illustrations of old muscle cars and sports cars and all the branding has been taken off and they seem to be ok. But just because they "seem" to be ok doesn't mean they actually are.
I guess the same would be the same for illustrating celebrity likenesses. Not only do you have the celebrity likeness to contend with, but also the copyright of the photographer. I often see people selling drawings of celebrities and I guess that would fall under a gray area as well.
You can TEACH it, you can show it... but can you sell it, is the question.
And even more broadly of a question... would having ads and monetisation classify as "selling" and "profit" enough to be an IP issue. Right now, no probably not, but it wouldn't surprise me if it could become an issue in the next years.