Support has to buy sightstone or get banned?

So basically i have been in few games now where people threaten to report me for not buying sightstone. As i like to play Zilean support very offensively because the enemy never expects a Zilean to do that much aoe damage i kinda rush my dmg items first and usually try to get sightstone after 20 min, and sometimes if the game is about to be over i wont even buy it. Now the question is, is that really a bannabe offence?
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