I don't think most people are inherently against boosting itself, myself included. Boosting has always existed in some form, whether it's helping out guildmates, friends or random players. In a lot of cases, it's just players using their extra time to help others while earning a bit of gold on the side. That part isn't really the issue for me.
What's really frustrating and I think this is where much of the community frustration comes from is the constant spam advertising in global, trade, and general chats. You often see level 20 characters sitting AFK in cities, endlessly posting boost ads every few seconds. Many of them don't respond to whispers, which raises some red flags. Personally I think it's botting, but they're using an addon so they get away with it.
That kind of spam clutters communication channels and makes it harder for players to find groups or interact with the community in a meaningful way. It's less about boosting itself and more about how it's being promoted in disruptive, low-effort ways.
In short: boost if you want, help others, earn some gold - that's cool. But the way some of it's being advertised crosses a line and affects the rest of the player base more than people realize.