A massive commercial artist releasing a song that features copyrighted lyrics from another massive commercial artist. I would think it just makes good legal sense to obtain permission to use said protected lyrics. Especially considering the scale of the release and courtney's reputation for legal control over kurt's music in the past.
Courtney Love has no reservations about suing anyone over using Nirvana material first and foremost. She also owns the rights to the songs and any use of them must be obtained legally through her.