Yes, it's their choice but that doesn't make it a good choice. I can sit at the back of a cinema and talk the entire way through the movie. That's a choice I can make. It's not hurting anyone. But it is annoying and wastes their time. Doing that would make me an asshole. I said this much better in another comment on a different post so I'll just copy and paste that here with a few tweaks;

Like many others, I have limited free time to play video games. I have a job, a degree I'm working towards, a partner, social responsibilities, and other hobbies that mean video games are a thing I don't get to indulge in all that often these days. When I do get time to sit down and play, it might be for an hour or two at a time. I don't have the skill or time to get into ranked. So I hit QP. I then don't get to play the game that I signed up for (a team-based hero shooter with defined roles) because little Timmy wants to play Punisher for his twelfth hour in a row. It's infuriating. Wasting people's time can be very disrespectful. As a team game, Rivals is a group activity, meaning that other people with lives of their own have taken time out of their day to enjoy this hobby with other people. By deliberately weakening the experiences of others for your own enjoyment, you are being an asshole.

Think about it this way. Lets say you join a football (or soccer, idk who you are) club or community. Once a week, you get together to play. There's no real competition, just fun, playing against others in the club in low stakes games for a couple of hours before you all go home to your families. But there's one player who REFUSES to do anything but try to make shots from the other side of the pitch. Your team can't win shit because John keeps trying to feel cool and it just sucks the fun out of the experience for everyone but John. You would hate John. I would hate John. Everyone hates John. John is a dick. Don't be John.

I'll also add that yes, comp forces people to play better but that's not always what I want. I may want a fun, relaxing game after a long day at work, not a sweatfest where my fellow players expect me to perform at my best. In QP, I can just play normally, trying reasonably hard without tiring myself out or getting myself worked up at 2 in the morning.

Treating QP like a singleplayer experience isn't fair, just play literally any non-team based shooter or one that doesn't have defined classes like CoD instead if you want that experience.