On Fun Theory; by a great, drunken Master of that conspiracy:

It seems that by placing danmaku under the spellcard rule, the rule limits the freedom of the user, but that isn't true. To be unrestricted means to be able to do anything. On the contrary, that means the immediate pursuit of the best, which in turn destroys variation. If one were free, they need to pursue only "the most efficient, the most effective." For danmaku, that would be one with no gaps, or the fastest and largest attacks possible. That kind of attack can't be described as danmaku at all. Therefore, in a world without rules, danmaku is nonsense.

-- Marisa Kirisame, in her Grimoire