It's much easier to control the amount of salt when you use unsalted butter, also, I find cream cheese to be a bit on the salty side so if you used salted butter they just may be too salty,

The recipe calls for the addition of 1/2 tsp. salt, so using salted butter would yield too-salty cookies. Cream cheese also contains salt. Also, in general, unsalted butter is the standard for baking.

Salted butters don't all use the same amount of salt. Therefore, substituting it is going to be way harder than just using non-salted butter.