I'll chime in here very quickly to relate my own experience. I find that I usually only get "fair" games in Tournaments. For a while, I only played exclusively in tournaments. Did I run into abusive lists and some jerks, once in a while. In my experience, I ran into abuse far more frequently at my FLGS to the point that I don't bother trying to get a pick up game in down there except against a chosen few. I found I got bogged down in rules arguments because people just flat out didn't know the rules. (A lot of them don't even own the rulebook or the codices except in pirated pdf form which they didn't even bother to read. They are playing some kind of game involving miniatures, but it isn't Warhammer 40K). I found that the people with their egos most tied to winning the game were much more likely to be local pick-up gamers than the average tourney goer.
Secondly, I like the modelling aspect of the hobby a lot. I love seeing other people's armies and well-painted models. On our 40K night at the FLGS, there might be 2 or 3 well painted armies out of about 40 people hanging out there. There are folks playing for years with stuff that isn't even primered. That's fine for them I suppose, that doesn't take away from their enjoyment. But it takes away from mine, when I try to get a game in, and someone is tailoring their list on the spot after seeing mine. "Oh you have terminators? I'm proxying stuff, all these flamers are really plasma guns, and these empty bases are blah blah blah". That gets annoying. I don't have to worry about that at a tourney. Everyone's stuff is painted. Most of them look good. Opponents can't tailor their lists before the game.
Third. I'm very busy now. It isn't easy for me to get games of 40K in. Most of my time is spent on the hobby aspect of the game. But I can take a weekend off and get 3 games in, in one day against some solid opponents. If I travel to a big event, that's even better since it means 5 or more games, plus some pick up games, plus I get to hang out with friends that I usually only see at the tourneys.
Now here's the trick most non-tourney folks don't know, because they're too committed to their stereotypical idea of the WAAC tourney player to bother finding out. Tourneys cost $$, national level events cost a lot of $$. If a person's only goal in going to the tourney is to win, they are going to be severely disappointed. There are so many things that go into the winning of a tournament that even the world's greatest 40K player ever can't bank on it. It is a dice game after all. If all a person was going for was to win prize support, for most tournaments, they'd be better off just buying the stuff outright, rather than spending money on traveling to the tournament. Most tourney players don't go there to win. Because we know better. We go to place well (hopefully), play a bunch of games a short amount of time, and hang out with like-minded people. (Wow, that sounds a lot like why people play pick up games at the FLGS, doesn't it?)
The game is about what one of my friends calls "Cooperative Competition". There definitely is a social contract when I step up to the table. Problems arise when people ignore that contract.
What I find a little sad is that so many people take pleasure in vilifying the tournament scene almost to the point you'd think we were enemies somehow. (just look at some of the responses in this very thread). We're all part of the same hobby and the same game. I happen to adore all the new releases and codices and dataslates etc. But I also don't want to spend a lot of money traveling to Vegas, or Adepticon or whatever, pull out my models that I spent a long time on, and then put them immediately away because someone just dropped a 15" Apocalypse Mega-blast on them.
It's all about expectation management. When Adepticon runs the Gladiator (which is a tourney that allows Apocalypse units), people don't complain about facing D weapons. If I'm playing at the FLGS, and someone wants to run a transcendant C'tan for fun. Sure why not?
Problems arise when expectations are violated and the social contract ignored.
One other thing before I leave you guys. I don't run into seasoned tourney players that are jerks. When I do run into a jerk at a tourney, it is often a person's 1st or 2nd tourney, and they are behaving that way because they think that's the way people are supposed to behave in a tourney. When I discovered this a couple of years ago, I found I could just step in and chat with the player for a few minutes and explain to them that playing a tourney game was no different than playing a friendly game. There was no need for arguments and being a jerk. Something to think about.