The theory was that although Kirk would recognize goggles, or protective eyewear or even sunshades---he wouldn't easily recognize the small, spindly Fraklin 'half-glassess' readily.
Of course, if he was alone he would have examined them and figured it out. But seeing as how they were a gift and McCoy was standing right there, he just says, "What is it?"
Not a mind blowing leap for such momentary confusion, IMO.
And yeah folks the ABC/Director's cut that includes the extended dialogue has been seen since 1982---you might want to check it out!