Wouldn't be an issue if SEGA had some sort of archive of their own assets that licensed devs would have to pull from if they were not making their own. Stuff like Sonic Speed Simulator definitely has to rely on asset rips from Models and Textures resource - their animal capsule model (when they had one) was a rip from the Statue Room in Sonic Generations and nobody thought to remove the base stand. As far as the issue itself goes, not everyone can have intimate knowledge or time to double-check whether something that looks official (many things do) actually is. I can't imagine scrolling 10 pages of results on Google trying to find official sources because you're given a "Good luck!" from the people who would normally provide those resources is a really good use of work time either. Even someone you'd expect NOT to fall victim to these issues - the guy working on the Sonic 3 Origins sprites - did.