Given the fact that many people who actually paid for their tickets are burning them or otherwise not showing up, I am not sure giving tickets away will make much difference.
If your protest against the NFL is sincere, you're not going to the games on principle, not because you can't afford the ticket.
There will be no shortage of BLM sympathizers eager to accept discount tickets. Gate revenues will drop, but there will still be a butt in every seat to make the optics work.