As noted in the thread related to "Black Spots on Quarters" you can have a coin that is in circulation but can still grade as a mint state (MS) coin, as long as there isn't any wear on the coin (minor slide marks or cabinet friction for some older coins aside).
For example, you could get rolls of coins from your local bank that come direct from mint bags which will invariably have some contact marks, which are not considered '"wear", but otherwise could still be in a "mint state" as long as the coins aren't roughly slid across a counter or otherwise handled causing wear. And for the 1972 Kennedy half dollar the "wear" you are referring to appears to just be contact or bag marks which are not considered wear in grading coins.