Holy moly, you went kinda hard at that other user, whilst holding only partial understanding yourself. It's like a movie, 'The Mysterious Case of the Overconfident Corrector' or some such.

1. There are more casing rules besides 'TC (Title casing) and SC (Sentence casing) in English grammar.

2. They are the most popular because both Chicago MLA and APA feature distinct rules for when to use them in the types of writing they cover.

3. There is no authority, grammatical or legal, that governs the usage of casing rules across the English language itself. They are determined by the bodies of authority in the medium they are being published. (e.g. No one polices which to use in coding. Google uses one, Apple another, etc).

4. Like any other style guidelines, the ostensible purpose for selecting one casing rule over another is to achieve a desired readability effect. Here is an example article going into how each one feels when using in UX design: https://uxplanet.org/why-letter-casing-is-important-to-consider-during-design-decisions-50402acd0a4e.

5. I agree with the other commenter's spirit. They chose poorly for a billboard, imo, and sentence casing would produce a faster and more retainable read.

I'm not angry lol

And also that only applies in a title, this isn't a title, it's a sentence