Q:

Why is the Ubuntu-for-Windows default shell a root shell?

From day one of using Linux we are told that running as root is extremely dangerous.  Here we are though on day one of introducing Linux to a wider audience by enabling it in Windows 10 we provide the user with root access.  I realize that it is sandboxed from the Windows 10 environment but providing a default root environment to all users (even standard users) is potentially dangerous for any system.  Why would we decide that this is the right course of action with this software?

A:

It's because user IDs are one of the several rough edges in this beta test level subsystem.  You are not, in fact, actually a superuser. 
It just looks that way because the Windows NT Linux subsystem reports it that way to Ubuntu applications through the various system calls.  The parts of the system call layer that deal with user IDs simply say that everything is UID 0.  
The Ubuntu program is running with your user process token and you do not have any abilities beyond what you could do with a Win32 program as yourself.
This is not the old (Interix) Windows NT POSIX subsystem, with its sophisticated mechanism for mapping Windows SIDs to POSIX UIDs and GIDs.  ☺
And this is in the release notes.
Further reading

Rich Turner (2016-04-06).  "Release Notes." Bash on Ubuntu on Windows.   MSDN Blogs.  Microsoft.
Charlie Russel (2004-04-22).  "Features of Services for Unix: User Name Mapping" Introduction to Microsoft Windows Services for UNIX 3.5.  TechNet.  Microsoft.
"User and Group accounts" Permissions In Microsoft Services for UNIX v3.0. 2003-05-01.  TechNet.  Microsoft.

