Oh I get what you’re saying but it took me a minute. Given the target audience (I presume) of the rest of the stickers, this person is likely saying “Why wouldn’t Black Lives Matter?” to the people against it, rather than saying it to the people in the BLM movement. I think the meaning changes depending on who the receiver is.
"Black Lives Matter" is frequently misinterpreted (either intentionally or not) to mean black lives matter more than others, thus the reaction "All Lives Matter." I think this sticker kind of disarms that reaction.