The "strike" doesn't being until you hit the restricted respawning zone- evidenced by the fact that in those beginning areas, there are often players running around who are not on your fireteam- once you cross that threshold, and it is just the three of you, then the strike has started- I join a playist to complete a strike, not run around in the open area before it killing enemies that do not help me acheive my goal- if you want to take your time and kill the enemies in the non-strike area, either get left behind and deal with it or go play patrol