The essence of simplicity includes picking files for the database filtering them as rapidly as possible. Hence, the Sansa ignores pdf files, executable files (exe), application specific extensions that are not needed my the device.
The SanDisk machine is also quite useful as a flash drive in either MSC or MTP modes, in addition to being a music device. Thus, I don’t see it as an oversight, but a design parameter. No need to bother with those files while doing its routine “database refresh”, a point of obvious contention with the average user.
Making normal operational tasks as rapid as possible is the goal, and filtering out files, via the file extension, is a great way to do it. Interesting find, incidentally. I hadn’t given it much thought.