This is all a matter of perspective.
When you hear the story, you are hearing that he got banned BECAUSE he played Singed support.
In reality he got banned because of the fact that he consistently forced Singed support on his teammates despite their objections.
Furthermore, the way he played those matches was so unconventional that his teammates were unable to adapt to the strategy, and most often left his ADC extremely vulnerable in a 1v2 lane. This crippled the bot lane and put his team behind, often with a loss of first blood and first tower, giving the enemy team a fed ADC and support.
So it was not the fact that he played Singed support. He could have played the lane conventionally and everything would have been just fine.
And it doesn't even matter that it was an off-meta champion. If he refused to play support as anyone other than Lulu, who is definitely accepted as a support, he is limiting his teams ability to form an effective strategy.
Also, if he had picked a conventional support and decided to play in the same way, he would also be creating a poor experience.
The fact that he intentionally and consistently repeated this behavior, even after having been temporarily suspended by Riot in attempt to get him to stop, is why he was banned.