If I'm trying to win a championship in the next three years and losing in week 17 this year will give me a better chance of doing that, I'm still trying to win, too. Why is it "unethical" to bench a player one week, but not similarly unethical to, say, trade away my first round pick to another owner? In both instances I'm making my team worse in the short term, at the exclusive benefit of a single other owner, in the hopes of making my team better in the long run.
Trading away players for future assets is a common rebuild situation where you are trying to improve your team. The fact it is worsening your team in the short term is part of that process. It is unavoidable because in order to gain future assets you have to give up current assets (at least this is a common way of doing so). You are not necessarily "trying to lose" but are hoping that's a byproduct. You are still playing the best players on your roster week to week at the time trying to put up the most points you can.
With respect to the benching your stud to try and lose week 17 scenario you are purposely trying to lose in the current year. It's a conscious decision to be worse than you are which isn't right for the rest of the league.
I understand the fine line and understand your argument. I just don't think it is the same thing with the latter (making an inferior lineup from your available roster) being unethical if a competitive situation.