I don’t know much about multiboxing tbh, and at the end of the day I don’t mind them that much, since there’s SO many ways in our times to make gold.
However, I can understand why people can be upset about it
First, the classes multiboxers use are all picked because they are really good at tagging a huge chunk of the area you’re in, while you (and possibly your teammates) probably play whatever class you main//reroll farm on. And so you might struggle a lot to compete in tagging mobs, and it can become frustrating after a while.
There could also be the fact that it’s one person privatizing a large farming area, as you can often see at the Blood Gate or the farmish place in Drustvar (as a skinner, I sometimes go there ; sometimes there’s a multiboxer, sometimes not), while you, if you team up, you’ll try to play together to make everyone’s day ; and in that perspective, fighting almost cluessly against a bunch of druids and not being able to tag mobs can lead to a lot of resentment against this multiboxer.
Also, I can understand that leveling people can get quite upset while trying to quest or doing WQ in Suramar (I farm a lot of Legion content, and sometimes you can litterally not do anything, it happens yes, but not that often).
But, if of course the dude is not botting, you can easily turn around the problem : go to another farming area, or just go do some Skyreach runs in WoD, you’ll probably make more gold than getting angered and not touching any mobs. You could also, as we’re in a MMO, just try to reach out to the dude, even group up with him if he wants to. If he doesn’t want, well that’s sad but it is what it is, just go try to get gold by other means.
Personnally, as I said, I don’t really care ; if they’re botting then it’s harmful, but if they’re not, they’re doing the best job they can to get gold without breaking ToS, probably making them happy, and at the end of the day that’s all what matters.
(sorry for the confused English btw, hope I made minimal sense)