I don't think that's the issue, at least not with all of the packs. The Remix Sessions for Lady Gaga and Dua Lipa are indeed not royalty free and those projects are for personal use only (Ironically, those are the ones I haven't had trouble importing). But as far as I know, the Producer Packs (Boys Noize, TRAKGIRL etc.) are royalty free and can be used in original songs even commercially, just like the Sound Packs and the rest of the content on GarageBand. Feel free to correct me if I mixed something up