1) they don't feel they can NOT police content
2) they are wayyyyy too big to police content manually
3) any AI they train to police content, is not going to be able to state clearly why a given post/account was banned, and anyway...
4) if you state objective, clear rules that are automated, it will be pretty easy to game them (e.g. using euphemisms or code words for objectionable content)
I don't see how FB can get out of this situation. Any social network that is as large as it is, which relies on advertising (and thus cannot be as content-neutral as e.g. phone companies are regarding what you talk about on the phone), is going to face the same situation.
Of course, if FB ends up getting way smaller as a result, that would be fine by me. But I'm not holding my breath...