Themes are very powerful beings in KDE. they can install SDDM themes and scripts, they can set Kvantum themes, custom parameters for other parts of the system etc.

You can’t really do that shit without scripting

It’s really not uncommon for a lot of themes to package an installer script, in case they have multiple versions, or multiple colors bundled. Realistically, they should just each have their own store page, but it’s a colossal pain in the ass. The Catppuccin global theme, for example, has 16 color variants, 2 decoration variants each, and then also a version with no splash. The whitesur theme is similar.

I do agree though that if this is going to continue to exist, it should not have permissions it has today

Because themes are not just skins, as I understand. Themes are a collection of a lot of different kinds of components, from color schemes and fonts to window decorations and to a custom interactive SDDM menu as the other commenter said.