See that's the rub. To actually tailor a list you have to be unsportsmanlike in your behavior to this game. Your are literally meta gaming against another player who has either no idea you're going to do it or the resources to counter it.
It comes down to the extent of how you're changing your list for whoever you're playing. List tailoring is not swapping out a few special weapons, or changing one squad for another. Tailoring is literally building your army around crushing one specific players list because you know that's what they play every week.
Not neccesarily. Nobody said that the person had no idea or didn't have the resources to counter it; you seem to have pulled that out from your rear.
I have a few friends who I play 40k with and with one we discuss our lists rather underhandedly before the game. I'm taking flyers? I'll try to give him the impression I'm running a horde army or something. He does the same and we both know this so it becomes a game of poker list-building.
Some groups of friends will tailor against each other. Some won't. In the group that do, it forces your lists to evolve into something really powerful in the end. If you play against, say, a 'nid player, a space marine player and a tau player then tailoring yourselves to fight one-another will leave you, eventually, with a list that can deal with hordes, CC armies, shooty armies, varied armies, probably flyers and FMCs.
In my opinion, tailoring is bad sportsmanship IF the other player isn't fine with it.