Sure, you can get a working, used 3-speed for $150 at a cheaper bike shop or on Craigslist / Ebay, but if you are new to bikes you might not know if it was in good shape. Cheaper used bikes at garage sales usually would cost $100 of labor, plus parts, to fix up at a bike shop, if you don't have tools and ability to do the work yourself.

And a used 3-speed bike will surely have scratched paint, a few worn parts, and some bad features like steel rims, weak brakes, probably heavy hi-ten steel frame and components. Public is selling a slightly lighter bike, due to some alloy components and the 4130 Chrome-moly frame and fork, and it has modern brakes, a modern Nexus 3-speed (much nicer for newbies than the durable but finicky Sturmey Archers from years past), and of course good looks.

$650 is pretty cheap compared to a pair of designer jeans or handbag, and very cheap next to a new Vespa scooter, or a new car. And one of these 3-speeds will be more useful and comfortable than a fixed-gear.

Personally, I went the Craigslist + upgrade route on two 3-speed bikes, but I've spent a couple hundred on parts and repairs (new Shimano 333 hub; they wear out; new shifting cable, new brake pads and cables, new chainwheels and cogs to lower the gearing, new grips, rack, basket....), and the paint is still scratched and rusty, and the bearings could use replacing...

I spent $850 for a Breezer 8-speed (with hub dynamo), which is great, but for many people a 3-speed would be perfect, if it is geared low enough. And I'm glad Public is selling some new 3-speeds that appeal to people who would otherwise be buying a fixie, or a Vespa or something.