They had a closer relationship with Fiat back when Fiat first appeared in the GT series right up to GT4, t's only recently that Ferrari bought Maserati from Fiat which may sound odd since Fiat owns Ferrari but it's not in the business world. One company owning or having a close relationship with another means nothing when licensing is concerned. There's as good a change Maserati appears in GT5 as any other manufacturer, barring manufacturers with exclusive agreements already in place such as Porsche who's chances of appearing are highly unlikely due to thier exclusive licensing agreement with EA.
If one comapny could be in simply becasue it has a close relationship wth another then we would have had Ferrari, Porsche and Lamborghini a long time agao, they all are owned/ work closely with other manufacturers. Lamborghini is owned by VAG, Porsche is art owned by VAG, Ferrari is owned by Fiat. It's all irrelevant in this situation.
As for the posters, I may be wrong about it being copyright, I'm not sure on the legality of them replicating another companies poster. It could simply be a different poster has been put up since the photo's for Google maps were taken and that's what PD have put in. Eitherway you could question why bother making thme at all, they're rather minor details, but either way that attention to detail is one of the things that makes GT special.