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.

You are misreading that

"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.

They're a little confused but they have the spirit

All lives matter is true, the message got hijacked by the wrong people, but it is still true that no matter your skin color your life matters and you should have the same opportunities and be treated fairly by authorities and so on and the message is inclusive of all communities facing racism, because black people might be treated the worst (along with First Nations if we're being honest), all non-whites are still victims and even whites of the "wrong" socioeconomic background don't get treated the same as well off whites.

Now, the sticker itself is just a way to ask the question to those against the BLM movement... "Please, explain why their lives shouldn't matter exactly?" You can't answer that question without being racist