They're linux ereaders in the sense they use the Linux kernel (not vanilla, vendor provided). Stock apps use the framebuffer directly (except Amazon, which has some sort of X). Binaries targetting the same ABI will work fine, except those that want to write something to the screen.
Any distro can be ported. The only change needed is including eink refresh routines into the
fbdev X driver. Touchscreen should work out of the box.
The reason almost nobody is interested is because the input/output sucks to be used as a computer, with computer programs. They're also very limited in CPU power, have no GPU, no wired USB host mode and lack features you would expect on regular computers.