Another big part of the issue is this leads to bullying and toxicity. Small example: 2 people pick Zen and Lucio. Someone goes: we need Mercy, Zen you should switch. "I can't play Mercy, but I can play Zen or these other damage if someone wants to swap roles." No, we need Mercy, change now. "Why don't you ask this person on Reaper if they can change? They have 100 hours on Mercy." Nah, Reaper is a good pick, change to Mercy or we'll report you for poor teamwork!
Another example: team with McCree. We need 76, he counters Pharah better! "I'm countering Pharah fine as McCree, she dies every time she's in the open!" Nah, I've died to her twice over the last 5 minutes, 76 counters better, swap before I report you!
Are these extremes? Maybe, but the bullying and toxicity to force a switch YOU (figurative) want is very likely.
Blizzard is also playing both sides and punishing people for it. Paraphrasing obviously, but from the recent blue post that was linked:
We do not count having a main as bad or poor teamwork; however, people should consider their team comp for their choices and build around it. (So, you already have a hypocritical statement there.). Another part is the statement "work around their teams picks." (It makes it seem like Torb is a reportable offense, but doesn't this statement also show that Torb could report his team as well for not supporting his pick and basing the team around it?)
Blizz is playing both sides with no clear cut boundaries and people (some, not all as trolls exist) are getting caught in the crosshairs from it. Yes, it is a team game, and picking Torb (in these examples) is not a guaranteed loss. On a personal level, I never assume a loss or get mad at people's picks as I've played enough games to see people pull all kinds of shit off. If it's not working, I'll ask if people can swap things up (nicely).