I am pretty sure there is a bit of an industry with law firms on the look out for any form of copyright infringement which they can make a case out of. They then take the proposed case to the company being 'infringed' and they tell them how much money they can make if they win with the firm taking a cut.
http://tcattorney.typepad.com/ip/
http://www.artlaws.com/
As mentioned (and back on topic): I believe that the factor of having to pay royalties to car companies has little or nothing to do as to why Hasbro doesn't release G1 toys. Those molds were already legally owned by Hasbro/Takara 25+ years ago. It's about collector pieces not being part of Hasbro's business model when it comes to retail.