I wondered about that myself. Oddly enought they are not tied down at all. You lay your strings past the ends of the loop serving and then the loop is served. I can only guess that the pressure applied by the serving compresses the bundle and the loop against the tip helps to hold it. Check out my thread on the endless loop jig. There is a link to a good site. I guess if one wanted to be AR about it the string maker could wrap the tag ends around the bundle a few times before serving but the way I explained it it certainly does not come loose.
Hope this helps!