@Marius L I didn't understand the issue myself, at first, until a couple hands like the examples below happened at a 1/2 NLHE game I played in. It was a home game, but a serious game. The average stack size was typically around $250-$500 as the night went on. Most, but not all, players were agreeable to run boards multiple times when all-in. In these examples, players are all in (just assume pre-flop for this example only), assume pot is same for each example below, and they're deciding to run it once or twice (or 3 times, if allowed). Player B was fairly new to the game, and may not have played in too many games where they ran boards multiple times.
Example Hand 1 -- Player A has KK, Player B has QQ. Player B asks to see Player A's cards, A obliges, B sees he's behind with QQ, and they both agree to run it multiple times.
Example Hand 2 -- Player C has JJ, Player B has QQ. Player B asks to see Player C's cards, C obliges, B sees he's ahead with QQ, and now Player B says just run it one time. Player C says - 'hey Player B, you just ran it twice when you were behind, but now that I'm behind, you say run it once!? Come on, lets run it multiple times" Player B doesn't budge and says run it once.
It just came off as poor etiquette on Player B, and I think most players at the table agreed.
After that hand happened, I recall one player in particular would tell newer players -- I'm not showing my cards, make your decision to run it multiple times or not irrespective of what
my cards are. I tend to abide by this thinking.
Side note - I think Player B later came to understand it how it was poor etiquette as well, and he turned out to be a good guy, it just rubbed people the wrong way at the time.