halacha - are pig bones (and feather from non-kosher animals, human hair, human dried-bones, etc.) kosher?

i was always told that inedible things are kosher (from the verse of sending the nivaila to the ger (bais yosef yd 103.1 מדכתיב לא תאכלו כל נבלה לגר אשר בשעריך תתננה ואכלה נבלה הראויה לגר קרויה נבלה ושאינה ראויה לגר אינה קרויה נבלה
(בכורות כג, ב)))

but today i read

Mishneh Torah » Sefer Kedushah » Ma'achalot Assurot - Chapter 4 - 18

When a person eats the skin, the bones, the sinews, the horns, or the hoofs of a nevelah, a trefe, or a non-kosher domesticated animal or wild beast, from the nails of a non-kosher fowl in the places where blood would spurt through when they are cut off, or from their placenta, although this is forbidden, he is not liable.

does everyone agree that it is forbidden to eat bones (without marrow) from a not kosher animal? (what is the final ruling? (theoretical question))

what i the source and reason for this prohibition?