Checking the option to Keep Me Signed in should keep you signed in for 2 weeks, however...The thing to remember is that arcgis.com and xxx.maps.arcgis.com (org site) are effectively different websites. You can be logged in to both at the same time, with an org account and a public account. The map service layer you reference is one of the Esri layers that require you to be signed in with an organization account. So, while you are signed in to your org account at xxx.maps.arcgis.com, the URL to the item is www.arcgis.com, where you are not signed in. Thus, you get prompted to sign in. If you were to replace the "www" your org prefix of "xxx.maps." in this URL http://www.arcgis.com/home/item.html?id=dfc9008829a6487db7e3050045c163fe
you would be able to add the layer without being prompted to sign in.Thanks,Mike