The assistants are not overruling the referee they give what they see and the referee then chooses whether to go with them, or not. With VAR it would be different. Referee waves play on but VAR then says "You missed a foul there". What choice does the onfield referee have currently?
1) Let the VAR make a subjective decision, that overrules the one he made, or
2) tell VAR thanks, but I have made my decision.
Using the monitor allows VAR to say "I think you MAY have made an error there" so referee then reviews it and makes the final decision. There should be only one person making subjective calls.
The way that PGMOL are using VAR the Law needs changing - every time the phrase 'In the opinion of the referee' appears it needs to be changed to 'In the opinion of the referee, unless VAR disagrees'