You can look at it in some ways.
1.
"Oh, this guy is kiting 3 ettins. Damn, he's skilled! Oh, one ettin about to die, tag him to get XP... now he's working on the other 2, oh he pulled another one! The second one bout to die, tag him again"
The player who is kiting must be thinking "Damn it son, help me out!", even though he also got XP and loot.
2.
"Oh, someone else is on the heart as well. Oh well, time to put some shoulder grease into it" ----> proceeds to help nuke the ettins that the player is kiting.
The player who is kiting now thinks "Thanks for helping out son!"
And then they both proceed to kill ettins. They might pull different ettins, but they bring them close to each other proximity, so that the heart can be completed faster.
Well, during all BWE, I always comes across the second situation. In fact, nobody was leeching off stuff, so the first situation is nonexistant. Since naturally, YOUR heart will be completed faster if you HELP the poor guy kills his ettins, instead of waiting for the last moment to tag and "killsteal".
And everyone got their own loot and XP, so why not help the guy, and why must you be aggrieved if someone else is helping you to bring down those 3 ettins you are kiting?