Subpixel rendering makes no sense on a tablet where the direction of the subpixels changes from landspace to portrait mode.

Couldn't the subpixel hinting change too?

You can, but the point of subpixel rendering is increasing effective resolution, and for English text, horizontal resolution is more useful than vertical resolution. See: https://www.grc.com/ctwhat.htm.

It's not just English text, it's virtually every written language. There are very few that use a vertical orientation, and fewer still that use only a vertical orientation.

Hebrew, Georgian and Arabic all benefit equally from sub-pixel resolution. Even vertical Chinese would be improved by having more detail on each character.