Boy, you have a lot of misconceptions about the Support role.
First of all, Sightstone is (as others have tried to say) not a Support-exclusive item. Sure, it builds into Support items like Eye of the Equinox, Eye of the Oasis, and Eye of the Watchers, but guess what; Face of the Mountain, Talisman of Ascension, and Frost Queen's Claim exist, too.
And, more than that, if we're just calling Sightstone a Support item just because their Support-Exclusives can build out from it, then why isn't it also a Jungler item? 'Cause, you know. Tracker's Knife exists. Bam. It is now also the Jungler's job to buy Sightstone, according to the logic I'm reading off your comments.
No, the Sightstone is something special because it enables the Support to "support" the team.
Is it so hard to see that warding and clearing vision is one crucial part of a Support? Without a Sightstone a Support just cannot fulfill that role.
I play Tank Supports. Vision is...Tertiary, if even that. I don't need vision to crush the opposition, or protect my lane partner. What I need is tools for engage and disengage, and I'm not going to drop money on a Sightstone to build into Eye of the Equinox when instead I could be better serving my lane partner with Face of the Mountain for survivability.
And that's just an example of the tactical options available to Supports. Sightstone isn't mandatory, and Supports can perform perfectly well without it.
If you don't have a Sightstone and just play any champ you want the way you want on Support you can just throw away the name and call it: "whatever you want, but you are not allowed to last hit".
Supports are still allowed to CS, believe it or not, most of us just tend to have self-imposed restrictions. Is my lane partner back in base? I'll CS where I can. Do I have good wave clear and my lane partner is half-way through, but the enemy wave is pressing our tower? I'll CS. While the bottom laner is present, we generally just hold back from CS'ing because, generally, they want their items faster.
(Also, Tank Supports and anyone who takes the Relic Shield Line kinda have to last-hit.)
Besides, amidst all of this talk of "what is the Support's job", nobody is going to ignore the greater issue here;
You're asking Riot to punish people playing one specific role for not buying one specific item.
That's insane.
Might as well be punishing any top laners who don't take Teleport next, huh?