Part of their problem is the historic precedent that they'll virtually always try to unlap the lapped cars - the teams now expect this behaviour.
The rules actually provide an option to avoid unlapping and arguably it would normally be far quicker, easier and typically fairer anyway to leave lapped cars exactly where they are. The issue is whether it would cause too many different cars trying to do different things close together from the restart.
It would also add an interesting tactical dimension to safety car pitting if the teams needed to consider whether they'd pit themselves behind more backmarker traffic or not, as they'd need to consider in normal race conditions.
In a scenario like this, having to lap 5 cars cut Lewis' advantage from a margin that could have afforded him an almost risk free pit stop, so from a 'fair racing' point of view it's hardly unfair that cars behind should expect to navigate those same back markers to be able to attack the leader - they've still been given some advantage in being brought right up to the back of that traffic anyway.
