Browsers are really not good at rendering SVG. Both WebKit and Gecko are riddled with bugs once you start using the non-trivial features of SVG. Worse still, browsers don't even support the latest version of SVG, which suffered a fate similar to ECMAScript 4. However, vector illustrating programs will happily generate files which use the browser-incompatible newer versions.
So while it may be possible to target SVG as a backend for rendering basic drawings, displaying arbitrary SVG files is, in practice, a lost cause.
They don't have to send the straight svg for preview. They could either modify it for compatibility or at least render it server-side to raster and send it over. It's not impossible, at any rate.
So while it may be possible to target SVG as a backend for rendering basic drawings, displaying arbitrary SVG files is, in practice, a lost cause.