This is team based though. You're more than welcome to go and practice heroes with your friends or AI but if you're playing online in a team then you should at least be playing the objective.
I don't want to belabor the point, but I'll point out again: From an abstract perspective, ask yourself why this is true? Who gets to decide how a player "should" be playing? With the limited modes currently in the game, and the issues with player vs. AI, there are some obvious motivations I listed for people to sometimes ignore normal teamplay.
Obviously this negatively affects some players, but a game isn't a system of morality where there's a strict right and wrong. Instead, a game is something that's probably intended to be fun, and what's fun is different for different people, maybe even at different times. If what's fun for the most people coincides with what a game encourages and what game modes are available, then great.
Imagine it as a sort of "fun economy". Blizzard can affect the experience by adding things like ranked mode, but ultimately players determine the experience by choosing what's most valuable (fun / ranking / whatever) to them, or leaving the game entirely.
Some people may want their own definition of fun to reign supreme, and all those who disagree to leave completely, but ultimately some compromise will probably be reached instead.
Then why the in the blue fuck choose Quick Play
Because the only alternative is vs. AI (obvious problems compared to playing against real humans) or custom (not enough people) or Brawl (obvious limitations).
Once ranked is added, then I'm more in agreement. Instead, it's casual meets competitive, and everyone is in the same melting pot.