In all honesty "we're not supposed to see that" as it's not "really there" or if it IS there a design feature of the uniform not the "zipper." As we all know zippers are obsolete in the 24c, they have "nanoseam" closures the clasp the uniform shut int he front. This is very, very nicely demonstrated when Ro takes off her tunic in "Ensign Ro" (further suggesting that there's a "nanoseam" between the top and pant of the female uniform to make it look like one piece. Meaning the two-piece look of the male uniform is a design feature.)
Ah, so basically they're supposed to be like those zippers around the neck we weren't supposed to see on the TOS uniforms.
Shame then that they were so incredibly obvious, even in SD.