That's only partially true. Porsche/Volkswagen don't want their cars "associated with war" -- and since the majority of Transformers series are associated with an intergalactic civil war, then yeah, they can't be associated. But there's been at least one exception to this and that was Transformers Disney Label's Donald Duck. It's a Transformer and it's a VW. This was obviously permitted because Disney Label Transformers are not tied-in with any other Transformer continuities which are associated with war.