Board Thread:Episode Discussions/@comment-36780105-20180831201857/@comment-9782468-20180913153736

The Chunin exams forbid the tool before the exam started, that's why it was a problem.

Kurama is a part of Naruto. If you remove Kurama, Naruto dies. If Naruto dies, Kurama does too (reforms after a long time but still). Their power is one and the same for all intents and purposes.

The tool is banned because it's haxed. People could seal whatever they want in there, with no regard to their actual ability. And their actual ability is what was trying to be measured by the Chunin exams.

Plus, it's a very unfair advantage. Having Kage level techniques to use with no chakra costs is just overkill in the realm of Genin and Chunin. It's the same reason why Sage Mode was banned.