Because tkinter.py uses exec to execute code from a file, it has a security bug. It searches for the file in the home dir. Apparently, on my system (don't know if its the same on others), the $HOME variable is the same as the non-root one when running Python with root priveleges
Steps to reproduce:
create a file called .Tk.py in your home folder
Whatever code is in that file will be executed.

run these three lines of code in Python:

import tkinter
w = tkinter.Tk()
w.mainloop()

I will add more details in later comments.