The article lays it out pretty clearly on why if you read it.
It is unclear how these AI-generated songs will affect the royalties Spotify pays to artists. The company is reportedly already negotiating with record labels, but how high or low the payments to artists will ultimately be is not yet known. Spotify is also reportedly struggling to persuade some artists to agree to the use of their music by AI. According to an analyst quoted by the Financial Times, around 8 percent of all Spotify subscribers could be paying this surcharge for AI-generated songs by the end of 2028.
Money, it's always money. That's roughly 25 million subscribers that would be predicted to sign up for AI slop at 5 bucks a month. Combine that with what I'm sure would be looking to pay the artists a lot less and only having to pay for computing resources instead of people, that's still a fuck ton of money for them.