I like this little bit from the article about people who tried to cut in line..
"As word of the shop spread on social media throughout the weekend, curious customers came to snap photos in front of the sign or pictures of the line of people wrapped around the small strip mall. Those who tried to cut to avoid a several-hour wait were promptly shamed out of line."
It would be pretty bad if this is allowed, even if they don't make any money from it.
Projects like this could mess a lot with the brand perception of course. But it would not be the first time that I would be surprised by US law. Like it tolerating housebrand products that look the same standing next to the original, or TV ads where a company mentions the name of its direct competitor. Those things are just not allowed in Europe because it obviously gives competitors power to influence the perception of another brand. Not that I think this particular action would be very harmful to Starbucks, it's probably just good exposure.