The Motte is that black people are human beings with inalienable rights who should be treated fairly. Surely we can all agree on this.
The Bailey is that police should not defend themselves against criminals if they are black*, that they should not arrest criminals if they are black, that any police shooting of a black person is automatically due to racism, that if blacks are arrested or shot by police out of proportion to their numbers it must be due to racism. These are much more dubious propositions.
If we are being subjected to a Motte and Bairly argument, then some BLM advocates are being dishonest.
I think the reason Black Lives Matter produced such a strong visceral reaction is that it comes across as a Motte and Bailey argument (http://slatestarcodex.com/2014/11/03/all-in-all-another-brick-in-the-motte/)
The Motte is that black people are human beings with inalienable rights who should be treated fairly. Surely we can all agree on this.
The Bailey is that police should not defend themselves against criminals if they are black*, that they should not arrest criminals if they are black, that any police shooting of a black person is automatically due to racism, that if blacks are arrested or shot by police out of proportion to their numbers it must be due to racism. These are much more dubious propositions.
If we are being subjected to a Motte and Bairly argument, then some BLM advocates are being dishonest.
*substitute your preferred term.