You're not obliged to build it or anything, it'll just show up in your rank. Without looking you up, I'm guessing bronze or low silver. It's not about being a slave, it's about being good at league of legends.
Vision is super important. If I could do my job in the other roles and build a sightstone, I'd be thrilled. Unfortunately, 6 item cap, and the job of other roles (occasionally excluding jungler) means they're obliged to build tanky/damage.
With the support, their champion is (/should) be chosen because of the utility inherent to their kits, so they have the freedom to build utility items like sightstone (which is even relatively cheap). They generally shouldn't build damage since they're underleveled and gold starved compared to laners, so they wouldn't be very effective at any stage of the game except ultra-late (the point where vision is perhaps most important since a single pick can determine the game), and they don't need to build as tanky since they're the support and shouldn't be focused unless they're very out of position (and if they're focused without being out of position, it probably means a won teamfight).
Edit: And people criticise other roles builds when they do it wrong too. For the majority of ADCs, if they're not building an IE, they're almost certainly doing it wrong. Most ADC's actually rush it so don't get much flak (...although it should be BF->zeal item->IE, not IE->zeal item), it's just low elo supports who don't seem to understand their role.