It is about tactical discipline and each players' role within the team unit. Salah and Vardy become the out ball for when their team regain possession. To facilitate that their teammates drop off and get behind the ball.
DCL is our out ball and James not tracking back just means we are a player short, and we have a huge hole on the right-hand side. Our full backs push on, so we end up wide open on the counter-attack.
Ok so lets look at some stats, rather tha opinions or perceptions.
Tackles won per game this season;
James: 1.4
Salah: 0.6
Vardy: 0.5
Interceptions:
James: 0.9
Salah: 0
Vardy: 0
The reality is, that James is tackling back 2-3 times more than Vardy/Salah and is intercepting the ball infinitely more. Combined he's about 4-5 times the amount of tackles and intercepts than the above two players.
Yet nobody in their right mind would consider dropping Salah or Vardy, for their relatively (compared to James) poor defensive contribution. I would have no idea, why would would drop someone for lack of defensive contribution who does substantially more defensive work than other players with similar roles for other teams.
James's defensive contribution looks worse, because his attacking work is so good.