So I'm watching DAG vs SAL and Salford are 1-0 up. In the 50th minute the commentator says "Salford seem to be taking bits of extra time doing things and the ref is all over it for time wasting"

So when is time wasting classed as time wasting? If your losing 1-0 and play a certain way you don't seem to ever be penalised for time wasting.
Playing that certain way again but in a 1-0 lead is likely to see yellow cards issued.
I seem to be remember being at Anfield and a Burnley player being booked for time wasting, it was extremely harsh I thought.

So is time wasting only a thing when a team is in a winning position?

A lot may argue this thread is time wasting however it is a another genuine "pisses me off" thing.

Pope was booked for time wasting in his first full game for the Clarets at Anfield!
How Norwich keeper wasn’t booked for similar offence at Turf Moor the other week is a mystery!

makes me laugh when the likes of Pep complain about smaller teams time wasting yet they are happy for people like Busquets and Fernandinhio make 20 tactical fouls a game

The rule on time wasting is clear.

It only comes into effect during the last ten minutes of a game, only applies to the side leading the game and only if that team is in the bottom half of the table.