there's only one way to interpret hairstyle

There is no subjective definition

Every linguist worth their salt completely disagrees with you. Language is a matter of individual experience, it works over our overlaps of personal understandings, and those personal understandings are never perfectly aligned (common understandings of words even drift all the time because of this!). You can call slapping an adjective to a category its own new category, and that's fine, but different people have a different understanding of the concept. There is no "objective" definition or even an "objective" experience of any kind, it just isn't possible, that's not how human brains do things.

A concept like "oxygen" or even "water" might have a significantly more generally overlapping understanding from a large amount of people. Our common education, upbringings, and interactions with other speakers make a lot of English speakers agree on that. But a concept like "hairstyle" is something that requires a lot of nuance, because different people have wildly different interpretations of what's included or counted as its own "hairstyle". Many hairstyles you see as different might be seen to others as one singular hairstyle, or something you see as one hairstyle might be seen to others as different ones. Different people may think very differently at how color, length, texture, shape, accessories, etc. make up hairstyles. Many people even think of head/face shape and bodily features as part of a hairstyle (especially in certain religious contexts). Just because you have a certain understanding of it, and your logic makes sense to you, does not mean it is the "correct" understanding.

The idea of "there's no subjective definition" is extremely prescriptivist and is a spit in the face of modern language/psychology/sociology science. It's unfortunate that this kind of BS is propogated throughout our education system by "English Language Arts" teachers... and is why people genuinely think that AAVE is "bad English" and why people who don't know shit about language constantly have stupid long-winded arguments about how "actually this common/standard usage or pronunciation of a word is wrong", thinking they can enforce certain usages on other people because they can speak a version of the language.

That being said, I think for that exact reason it's absurd that there's even an attempted legal argument about length not being part of hairstyle. What somebody constitutes as a hairstyle is unique to them and the cultures they're a part of, and it's completely unreasonable to dictate that something they and their peers consider a hairstyle isn't a hairstyle, then punish them for it. It is literally their head hair. Same thing with facial hair and body hair. They can do whatever the hell they want with it.