Can't adjust line spacing in book bought from Kobo

I understand that I can't necessarily override the default formatting on sideloaded ePubs. (At least not without removing DRM. But presumably it's not really a DRM issue otherwise you wouldn't be able to read the book at all? Maybe fixable in firmware?)

But, I have a book which I bought from the Kobo store where I can adjust the font, font size, margins and justification - but not the line spacing.

Is this a feature or a bug?