While torrenting isn’t an illegal activity, downloading or torrenting unsanctioned copyright materials is illegal. In the same light, while RARBG isn’t an illegal site, the torrent site contains a lot of copyrighted materials that are downloaded illegally.
Because RARBG is banned in most countries of the world, using the site in such countries is not only illegal but a criminal offense that may attract severe penalties depending on the laws of the country. For instance, ISPs and copyright trolls are constantly checking peer-to-peer networks including RARBG, and can take legal action should a user be found to be torrenting copyrighted materials.