I don't think OP means ranged - cowards - people shouldn't shoot at all at people engaged in melee. But sometimes, it's complete obvious a glorious duel is going on, for example :

"HEY LOOK 2 DUDES IN THE MIDDLE OF NOWHERE ARE HAVING A MELEE FIGHT WHAT ABOUT I END IT EARLY WITH SOME LMB DERP?"

Any dude thinking this is ok clearly has no idea what is glorious 40k, or is a Tau - for what it's changing - at your own convenience !

Unless they're skilled.

Or they're wondering why you're off doing that instead of taking/defending a cap point.

Prob because as you're melee and let's take the case of a JPA to make it explicit, YOU DON'T NEED TO BE RIGHT ON THE CAP POINT, you can be usefull elsewhere, catching people where they don't forcibly expect to be caught, so here you are, happily running, trying to spot the next group of fools coming, and you see some derp ranged people escorting a melee dude, you sneaky slaughter the useless ranged people because they anyway don't deserve to live, and then finally PROFIT GLORY MELEE AND LET IT GO !

Pretty much every time I'm in a dank 1v1 honor duel or whatever I'm just delaying the guy so my team can shoot him (or dicking around). Melee classes aren't the counter to melee classes.

I'll stop shooting into gnarly melee duels like that if melee bros stop stabbing me in the back when I'm exchanging fire with the enemy.

For the most part I agree and tend not to fire into melee fights just so I don't down a friendly but there are a lot of times when you're on the side or behind and you have nothing but clear shots, why not take them ?

Heavy bolter until nothing is standing, to be safe...

If you run to melee three or more people and I have a charged plasma shot ready, I promise you won't feel anything. I don't care how skilled you think you are at spamming quick attack.

Or maybe they just want it to end so the guy can get back to doing something useful for the team.

Experiences differ for every player for every game. If the enemy is in shooting range than devastators should use their ability to fire into the opponent while the melee defend their backs and hold for close combat if the opponent gets too close. The melee unit (shoulder to shoulder) with a devastator shouldn't run in front of the ranged attackers when the opponent is 50 yards away to confront them with melee attack.
Yes, the server lag may be a strong issue with failed assists.
Yes, some players just don't care.