I read an entire thread on this topic somewhere not long ago, and as you can imagine, there were more "reasons" for the dots than you could EVER imagine! I remember one individual even saying his tint guy sanded them down so the tint would be applied better.
They are to shade the top of the window kind of like the blue tint on the top of the front window. Most tint shops will scrub those down with a green scrubby to help smooth them out and tint over them. The place that did my DD did that the tint is still on there and it looks great.
Q: Several years ago, black dots started appearing on new vehicle windshields. The dots are usually only around the periphery of the windshield. What purpose do these dots serve?--Brad, Midland, Texas
A:Windshield glass contains a black enamel band (called the frit) around the periphery that is baked into the glass. This black band includes a border of dots. See figure. The band has an etched surface to enable adhesive to bond to the glass, says a sales manager at Able Auto Glass. When car manufacturers install the windshield, they bond the windshield to the vehicle with adhesive placed on the etched part of the black enamel glass area (inside surface). The outside of the enamel band shades the adhesive from the Sun and protects it from ultraviolet radiation. The band also serves a cosmetic purpose by covering up the adhesive and gives the windshield edge "a more finished look."
[Steve Coyle, Auto Glass Specialists] The black dots blend the border into the glass
Rather than a straight black edge, the dot border blends the solid band into the clear glass, says Steve Coyle at Auto Glass Specialists. Some windshields also use the dots as a third sun visor to block the Sun behind the rearview mirror where most visors don't reach.