The test case appears to only fail with OS X Cocoa Tk (tested with Tk 8.5.17 and 8.6.3); it does not fail with the older OS X Carbon Tk 8.4.20 nor with X11 Tk 8.6.3.  It also fails similarly with Python 3.4.x.  FWIW, there have been other reported problems with Tkinter SimpleDialog functions using Cocoa Tk, for example Issue16023, and a number of other issues unique to Cocoa Tk.  These kinds of problems are usually not simple to analyze, requiring tracing events through Tkinter and Tk and possibly lower-layer OS X APIs.  It often helps to eliminate Python and Tkinter from the equation by trying to reproduce the problem with equivalent Tcl code in the Tcl/Tk wish shell if anyone feels up to it.