There is no such limitation. You might be confused by the difference between "Installation" and storing files. As you can see from one thread you participated in: Other standard programs running on Android have no trouble using files on the SD-card. For instance, I have Bittorrent Sync storing (syncing) files on my SD card which the 1Password app reads and writes to. This folder can be chosen at will, just like I would want to. Should application related files like notebooks be treated differently, and be constrained to the temporary cache folder on the SD card, then again: no problem. All we need is a checkbox saying "Use SD Card for offline notebooks" - where on the SD card they would be stored is not of importance.
I carry with me multi-GB notebooks, where each note could be a few MB due to pictures and pdfs (for instance food recipes). These are no fit for 4G connections with usage restrictions, and so I have opted to make them offline. Unfortunately, this consumes all available space on my 16GB Android phone (Xperia Z2), and so I have bought a 32GB expansion card. Now customer support is telling me I cannot manually set the download location! This should be user configurable - either on a per-notebook basis, or as a general setting. So I am raising this as a improvement suggestion for the developers. Other threads dealing with this problem: ThisSelectable offline folder path?