Those quick action clamp bolts have very many other uses. When you park your bike in a bike rack, you can turn the bars to make more room for others and get your bike in and out more easily when there are overlaps. When you lo k your bike to a tree or lampost, it makes more room for pedestrians. It makes a good anti-theft device, especially if you leave the clamps unlocked. A potential thief probably wouldn't be able to figure out why they can't steer, so they'd probably abandon it straight away. If you left the bars flat, they might assume that they need tools to straighten them to ride it. Every little helps.
I've seen a video of a thief trying to steel a bike off a rack from a car in London while the car was driving. He was following on a cheapo bike, and every time the car stopped at traffic lights he undid more of the clamps and straps. His planwas probably to abandon his bike and ride away on the valuable stolen one. With bars clamped flat, he would know that he couldn't ride it, so wouldn't have bothered.