Yeah that is not acceptable. One shouldn't have to change their behavior just to be a part of the event.
Bad bad bad on Riot's behalf. They're pissing off a lot of people with this and making them NOT want to do the missions, which is counterintuitive to them putting the mission system in in the first place.
They are literally driving people AWAY from using an aspect of the client they spent time and money to develop. That seems like a lose/lose to me and not very good marketing or deployment of this new feature.
Which would you rather have? People participating in something just by playing the game and getting stuff and being happy,
OR
People not participating because they don't want to remove an icon they probably paid good money for, and they system being a flop because of poor design and execution.
My pick would be participation across the board. Rather than pissing people off.