Just to be clear. Plagiarism is not the basis for this suit. Plagiarism is the direct copying of material without attribution to the original author. Plagiarism per se is not illegal in the US, and more commonly is an issue in academia. What is being claimed here, to the extent that it involves protected content, is copyright infringement, which is illegal, under the theory that the subject work involves making an unauthorized derivative work based on the protected original. That is not an easy thing to prove if the idea or conception is being exploited, as copyright only clearly protects the specific expression (words, art etc.) and not the general concept. The clearest case would be if someone makes a movie or a sequel that is clearly based on a novel, using the same characters names and attributes. A sitcom based on a group of characters who meet in a bar, would be a difficult case to make for the creators of Cheers, unless the similarity was truly striking. Lack of originality is, luckily for pop music composers, not actionable.
The easier case to make here revolves around the expectation that the pitch was an offer for sale of those apparently well developed ideas with the expectation that rejection of the offer was a rejection of the ideas being pitched. In that case, using the ideas without payment is a breach of contract, or a breach of confidence, which are also asserted. Unfortunately, the remedies available are more limited. A copyright infringement, would potentially subject the infringing party to an injunction preventing them from releasing the product, and thus put enormous pressure on them to pay up. A contractual dispute would be harder to enforce with an injunction.