I would argue that
Iluvatar was open to the imaginings of each of the
Ainur. He didn't dictate exactly what each of the Ainur played, he assigned them a part and a basic melody. They were supposed to then give that main theme their own variations and thoughts.
This could be compared to an orchestral piece from the Baroque era. Composers usually wrote out the main theme and added repeat signs. The performers were supposed to add their own interpretation and ornamentation to keep it from getting boring and overly repetitive. This allows for huge amounts of creativity and innovation, but the players still have to stay within the main theme given by the composer! It's also a huge responsibility; their are rules of music that must be kept in mind and the player cannot break them. But
Melkor wasn't happy to just add his own variations, he wanted to write his own music that was completely different. Writing his own music wasn't wrong, it was the time and the place that he chose for it. If a french hornist decided in the middle of an orchestral piece that they wanted to write their own music and started playing their own thing right in the middle of the piece, the conductor would have a spaz attack! But Iluvatar didn't freak out. He gave tried to change the theme so that it incorporated
Morgoth's ideas, but then Morgoth changed his part again to something else that contradicted the music. Iluvatar tried several times to include Morgoth's creativity in his theme, but Morgoth insisted on changing his part every time. This is not creative thinking, it's blatant rebellion against the will of the composer, Iluvatar.
And who could blame Iluvatar for insisting on order? What composer wants musicians to take a song that they wrote and completely change their part into something that doesn't fit into the overall plan? Heehee, Beethoven would have been absolutely furious! (but then, he was known for his temper

) I compose music and there are specific things that I make very clear how they should be played/performed, but there are other things that I allow each performer to interpet for themselves. Any good musician understands that it is only permissible to take a certain amount of liberty with someone else's work. Morgoth overstepped those bounds by trying to re-write his part entirely into something completely different from what was given to him by Iluvatar. And, given his knowledge and power, I'm sure his part was pretty important and impressive to begin with. Why did he have to insist on doing something completely different?