There is a lot of variables in play allowing you to get your primary role most of the time, so you can't say people "just like to complain". I put my roles as primary ADC and secondary MID, at my low elo I tend to get ADC around 80%-ish of the time, due to mid lane being more popular I assume. However regardless of where I put support, I get thrown into support 95% of the time. Even when i've queued with friends that have support as their primary and it's either my secondary or even FILL is my secondary, it STILL gives support to me instead sometimes lol. Like the time I queue'd with 3 friends and me and one other did bot-support and support-bot as our roles, i've still gotten support over him a few times even though we're 100% guaranteed to go bot lane together and it's my secondary lol. Luckily we just trade roles, so it works out in that situation.
So it really comes down to what times people play, what elo they are at can factor in as well due to role popularity, and then the luck factor I guess. I just think there's too many factors involved for anyone to say these people are just complaining. They all have a right to complain because something does need to be done, this many people wouldn't be upset if it wasn't a constant occurrence. It doesn't even technically effect me in most situations since I tend to get my main role more often when support or fill isn't one of my choices, but I can still see that there is an issue that needs work.