The problem stems in part from this:
Some say "Legal = fair".
Others say "Fair = balanced power", with 'balanced power' being a subjective judgement of equality and how best to get there.
The last element is some saying "Unfluffy = bad".
All of these points have their flaws, since the first is somewhat lazy and disingenous, while the second is impossible to argue as any kind of absolute truth, and the third suffers from the same problem but isn't even engaging with either of the others on common ground.
What it really comes down to is how the player views the competitive element in the game. If they see it as a pure contest for victory, using anything within the rules to get that victory, then they'll go with the first argument. These players view list-building as an integral part of the competitive element and will try to dominate in this aspect as well as the others. They are more likely to be accepting towards tailoring.
However, other players, those who say 'Fair = balanced power', are usually more interested in the competition in individual games rather than that in list-building. They favour games with armies of roughly equal power, and winning through luck and tactical know-how alone, rather than through having a big advantage in their army itself. They consider trying to get an advantage through list-building to be a cheap, and some will consider it a part of the game that ultimately spoils other aspects too, like variety, originality and stylishness. This group will reject list-tailoring as unfair and usually reject the most powerful possible lists too. Their baseline for 'balanced' is usually something arbitrary but considered and reasoned. (Personally, my baseline is this: the most powerful 2k lists for Orks, DE, Necrons, pure WH and pure DH should be the rough guide for 'fair' power. 2k, because any army can cheese up at 1k, and those armies because they have no outstandingly powerful combinations at 2k. So crazy stuff like Siren Bombs and 12 AssCann lists wouldn't make the cut.)
Lastly there are those who consider the RP aspect important and say that armies should be fluffy and be used fluffily. This can never be presented as anything more than personal taste and frankly it's pointless to try otherwise.
None of these views are absolutely right or wrong. There's no harm in trying to convince people to share one's view, arguing as passionately as you like, but ultimately there's no absolute justification for any one of these things. You pretty much just have to accept that they're preferences. In practical terms, the best thing to do is talk to opponents.
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Another problem comes from people who say "Nothing in the game is overpowered". This is cited either in blind faith in GW game balance from somewhat naive players, or from more canny players, those who say "Everything has a counter". Unfortunately, this last justifcation won't fly with anyone from the 'Fair = balanced power' group because they reject list-tailoring and favour evenly matched games. Again, two ways of seeing one situation, both based on a preference.
