Why does Adobe Premiere CC change my "Ease Out" keyframe interpolation to "Bezier"?
This is the expected behaviour. It's a bit confusing, but as you can see in your screenshot, the Temporal Interpolation context menu is split in two parts. The part above the seperating line (see your screenshot) sets the keyframe type. From the documentation:


  
  Linear: Creates a uniform rate of change between keyframes.
  Bezier: Lets you manually adjust the shape of the graph, and the rate of change, on either side of a keyframe. You can create very
  smooth changes using this method.
  Auto Bezier: Creates a smooth rate of change through a keyframe. As you change a keyframe’s value, the Auto Bezier direction handles
  change to maintain a smooth transition between keyframes.
  Continuous Bezier: Creates a smooth rate of change through a keyframe. However, unlike the Auto Bezier interpolation method,
  Continuous Bezier lets you adjust direction handles manually. As you
  change the shape of a graph on one side of a keyframe, the shape on
  the other side of the keyframe changes to maintain a smooth
  transition.
  Hold: Changes a property value without gradual transition (sudden effect changes). The graph following a keyframe with the Hold
  interpolation applied appears as a horizontal straight line.
  


The two options below the seperating line – Ease In &amp; Ease Out – are rather like presets or shortcuts. They switch the keyframe type (since easing in/out requires bézier curves and is therefore not possible in Hold or Linear modes) and set a smooth transition. If you open up the graph view in the Effect Settings panel, you can see the effect the two options have:



Linear.



Ease In. 



Ease In + Ease Out. 

You can still adjust the velocity interpolation manually using the Bézier handles seen in the screenshots above.