I don't think the intent is to do stuff in R, but perhaps to do stuff in arc* that uses R functionality. The arcpy interface is written by esri and can't be used in standalone (ie you need to have arcmap/pro installed). I suspect the R interface is similar to allowing people to use numpy, scipy and matplot within and arc* installed environment. In fact, much of R functionality can be done with other python based packages, such as the aforementioned.