The thing to bear in mind, by default, the 3D machining routines will try to make the full model you supply.
So in this example it will try to cut the outsides of the shape as well as the top curves.
This is why you get the plunge moves at either end - as it cuts the outsides of the shapes.
To avoid cutting the outsides, the easiest way is to set a negative Boundary Margin.
In the attached example, I set a Boundary Margin of -0.2, so the toolpaths are restricted to an area within the top surface.
The horizontal scanline routines are not terribly efficient for use with roughing out a shape.
They are designed to scan backwards and forwards over every part of the model, decreasing the Z by the depth increment if multiple passes are used. This results in higher areas being recut as you go deeper which is often a waste of time.
So the other thing I have done in the attached file is to introduce another 3D machinining operation that uses a waterline roughing pass. Waterline toolpaths are a lot more efficient for roughing as they will only cut stock once.
The waterline roughing pass also has a -0.2 boundary, so the toolpaths don't try to cut outside the shape.
The roughing pass has a Roughing Clearance of 0.05 which will leave this amount of stock uncut, for a finishing
pass to then clear.
The horizontal scanline pass can then have it's depth increment set to 0, so it acts as a finishing pass, just covering the surface once and giving you a good finish.
I set the toolprofile for the finishing pass as a ballnose and reduced the stepover to 0.2 which should help give you a better finish as well.
Bezier surfaces, are currently treated the same way as all 3D meshes. Being imported as STL or 3DS means they will be represented by a number of triangular facets. The more faces, the more accurately the surface will approximate the original curves. The number of faces should be defined when you export the the model from your CAD program.
The 3D operations will either calculate slices through the model (for the waterlining methods), or test height points along scanlines for the horizontal and vertical methods. Keeping the Resolution and Step Over parameters of the 2D operations small will result in the most accurate scanline toolpaths but can result in large processing times.
3D surfaces currently can not be used with the 2D machining operations (profiles and pockets), although it is possible to create plane slices from them with CamBam which will produce polylines that can be used for 2D machining.http://www.cambam.info/doc/dw/0.9.8/cad/EditSurface.htm
I hope this informations helps.