Well let's say that it's near the end of the game, both players have raxes done, it's only the shrines. One of the teams makes a push and while it's looking good, suddenly they lose because someone decided to backdoor them. That is really frustrating to see.
To put it in real warfare, it's like attacking the small villages, poisoning water, or donning enemy uniforms. It's just something you don't do.
As for my opinion on backdooring, in rare few cases it might be acceptable, such as hour thirty+ games, but most of the time? No... And I'd prefer if towers were able to hurt clones for Siki's ult, because a good Siki player will make an invincible clone that can probably do halfway health before the opponent is actually able to hurt himself enough to make the clone disappear. That REALLY cheeses me off.