As of Windows 10 Insider build >= 16215, you no longer have to enable Developer Mode (Settings -> Update & Security -> For Developers) in order to run Bash on Ubuntu on Windows!
This restriction was first put in place to safeguard non-technical users from inadvertently using a feature that was, at the time, very new and not yet widely exercised. Over the last year, two major releases, and many updates later, we're now much more confident of WSL's safety and utility and want to enable more users to take advantage of this valuable toolset.
You will still need to manually enable the Windows Subsystem for Linux optional component (via "Turn Windows features on or off"), in order to install WSL & its tools, but once installed and you've rebooted, you'll be able to install & run an Ubuntu instance without first enabling Developer Mode.
This change also ties into the new multi-distro scenario we announced at Build 2017 and which will be available very soon 🙂 More news on this front to follow!