It happened to me 5 out of 6 times in the last 3 weeks when I entered the SEA "BR" and they asked for my BP. So at one time I asked the agent what happened if I flew with another airline and didn't have an AS BP. The agent told me I would still be welcome in the BR with or without a AS BP. She told me the scanners were new and they wanted to use them on BP to make the process faster.
Yesterday I used my membership card at the old BR between C and D concourse after I landed, and the agent just swiped my card and let me in.