A certain amount of challenge/risk is needed by everyone to avoid boredom. Otherwise one could stay offline shooting the target drones and save $15 a month.
However, the are so many random variables in MA play that the odds are often completely incalculable. If you consider yourself to only be playing against the nearest thing with a red icon then it looks easy to tell whether it is a "fair" fight or not, but the MA rarely allows you the luxury of that single bandit being the only factor you have to allow for. And the Main Arena is such a wide-open sandbox that it is difficult for outsider to even say which game a given player is playing, much less whether or not they are playing it with even odds. For instance, at any given moment a player might be playing to get in one good fight and wax a tail if he can. Or his objective might be trying to get some kills and survive the myriad forms of death the MA has to offer in order to land them, or he can be trying to capture or prevent the capture of a base. If a guy desperately fighting to save a base shoots you in the face in order to get past you and strafe troops, rather than engaging in an ACM chess match, is it being unsportsmanlike or is he simply playing a different sport than the one you wanted him to play with you?
I think the perspective of most saying that certain ACM is unsportsman like etc. Is because when most like to fight or compete it's on even terms, and then the one to win in the 'even' fight is the victor. While that is a good way of fighting, I disagree when it's said to be the 'best' way.
I think a true fight, or at least one to be more proud of, is winning when the odds are against you, rather than even. Where the enemy is taking the 'cheap shots' etc, and you still come out on top. While it is annoying at times, it is still a legitimate tactic/tool that can be used in combat.
Some have a 'code of honor', myself included, a way of limiting how they play and defeating the enemy in those guidelines/rules. For example: when I first started playing, I did anything necessary to kill the enemy. As I got more experienced I challenged myself, one of the 'limiters' I put on myself, was not going Head-On when there is another way to kill the enemy. I will go Head-On, but only as a last resort. That being said, many make the simple mistake of trying to project their 'way of honor' or guidelines/limits on other players.