So why are people allowed to ruin the support role by going non-supports?

because people want to have fun

Because not playing meta isn't bannable

Well, this is a bit deeper than just one title.

There's regular, trolling, and off-meta.

An example of regular support would be building normally and playing normally. A champion who is established as a support doing support things.

An example of off-meta support would be building something like tanky support items with perhaps a spare few solo tank items . They're actually trying to play support with a champion not naturally classified as a support. Even if they aren't doing well, it doesn't mean they're trolling.

An example of a trolling support would be . Sure, you can actually play an effective Teemo support (mushrooms make for an anti-gank/vision tool, blinding dart keeps enemy ADC from doing much, and everyone focuses Teemo) but most people don't play a real Teemo support (if you do, good job). A Teemo who is trolling is probably building full AP and attempting to steal all your ADCs kills and cs.

The trick is to know when they're trolling and when they're off-meta. Reporting someone for experimenting is a jerk-move. Reporting someone for trolling is proper.

What champions do you consider non-support?

its a video game. get over it.

Because no one likes to support due to always getting blamed for losing, not a lot of impact, it's boring, you get 0 acknowledgment for your plays for being the reason for a big play, you get blamed for not having infinite cds and shields and heals.

So in the end you could've asked to switch

I would rather have a non-support than a sona support

The support meta is gradually changing as people are starting to acknowledge that a whole host of champions not normally seen as supports are actually playable there (thanks to season 4 gold-flow changes) and plenty of them are actually good at it too.