They give 1BTX to all registered addresses so now every registered address have at least 1BTX to take part weekly airdrops.
The 1 BTX didn't seem to go to ALL registered addresses; I have a registered address with 0 balance, and it didn't receive the 1 BTX today. So maybe it went to addresses with non-zero balance, or maybe just to addresses that had at least 1 BTX as someone else said.