Software companies just slap a ridiculous and wordy EULA on a product and say, "we're not responsible for anything that might happen with this software." Of course, Gmail isn't likely to kill anyone if it crashes. When carmakers let out a beta, like the original Cadillac V8-6-4, they quickly get sued into submission.
As for customers contributing to design, they do that with the dollars they spend on cars. There's a good reason why most mainstream cars tend to converge on a particular design ethos (read: bland) or function set: because that's what most people buy. Google's products are really cool, but aside from search, maps and AdSense, most others, including Gmail, actually trail far behind their competitors in market share. Being in perpetual beta may get the attention of us bleeding-edge geeks, but most regular people don't buy it.