I'd really rather that Starbucks encourage its staff to recognize customers as they walk in, rather than glance over whilst concentrating on the drive-through traffic. The walk-in trade, after all, might want to hang out for a while, and make repeat purchases, and leave a bigger tip.
Recognizing walk-in customers by attempting to start a conversation about race relations ... bad idea. Polite thing to do might be to simply say "no thanks," but if you want to encourage critical thinking, here are some suggested conversation starters from Charlie Sykes. Yeah, they're provocative. The last one, though, might be a way of keeping the line moving. "I’m sorry, I didn’t realize there were so many people behind me in line. Of course, I can move on. Thanks and #HaveaNiceDay."