This is mostly implemented, actually. Of the privileges you earn over time, the only things that are not hidden from the UI are, implicitly or explicitly:
- Vote Up
- Vote Down
- Create Tags
Everything else is hidden in some fashion when your reputation is insufficient. The option to "close" isn't visible. You don't get the split vote count tool tip. Bounty links will never turn up.
Create Tags is only there because you can technically try to create tags in the normal, usable tag field. But it's only implicitly available - it's not explicitly shown in the way that the voting buttons are.
So why show the voting buttons while at insufficient reputation? Because they are a core part of the website. The presence of the buttons, before you even earn the privileges, tells you that post score goes in both directions. It tells the user that it is possible to rate positive or negative, and consequently that their own contributions can be rated in either direction. Because voting is one of the most crucial aspects of the site, it is important to have it be immediately visible without even needing to dig into the FAQ.
Not showing either button, or only showing upvoting when your reputation is between 15 and 125, may mislead a new user into not knowing about downvoting. And that can be a harsh surprise if or when it happens.