Because the interface hasn't been defined yet. They're making experimental functions with no standard interface, so the prefix indicates it's not designed for production use yet. Look at CSS gradients, and even border-radius: until relatively recently, there were multiple different ways to achieve the same effect depending on the browser and that vendor's prefix: -moz-border-radius-topleft vs -webkit-border-top-left-radius (the latter won out and is now the standard, sans prefix) further complicated by non-circular radii; see http://www.css3.info/preview/rounded-border/. And that's dead-simple compared to gradients - look at the output to get something cross-browser here
This is definitely a real problem. Too bad the proposal doesn't actually solve it. While it goes in the right direction by asking for support of non-prefixed properties from day one, that effectively ensures that the first implementation out there defines the interface, which can easily leave a huge mess behind (again, see webkit's gradient interface). The proposal's author is aware of this and dismisses it as a non-issue, which is incredibly ignorant.