Let me start by saying, I absolutely hate one tricks and condone flex play.
The issue with off meta one tricks vs meta one tricks is the viability in team comps. Playing symmetra/torb every game can often be detrimental to the team, thus them generally getting frustrated with you and reporting you for playing that hero. (which is unfair to a degree, though at times you should be swapping with the team in mind because otherwise that's generally selfish and selfish behavior doesn't belong in competitive overwatch)
When you go to a character like tracer or Genji or even mercy (one upcoming example won't apply to the latter mentioned hero), they're so meta that they can work in almost any team comp, and it's because they were designed with that in mind (yes, you can play tracer in any team comp as her kit and play style work on any map and it's independent enough to not have to rely on synergies to perform well while having the potential to synergize with other characters at the same time, genji isn't exactly the same but can work in almost any comp and mercy you can play in any team comp no matter what and essentially synergizes with any character), so you can get away with one tricking these characters without impacting the team negatively for doing so (unless you play bad or someone takes your ot character).
What happens though is that one tricking can potentially cause a detriment to anyone if their character is taken by a stubborn player or another one trick, which can lead to that player swapping onto an unfamiliar character or throwing the game. Both of these are potentially detrimental to the team (except in tracer's case as playing her requires hitscan skills and you can get away with swapping to soldier instead at a smaller cost to the team). It's a behavior that causes problems in the community and general toxicity as a whole and frankly, though you can play any character you want, blizzard needs to be more clear as to what is reportable and what isn't.
