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Option 1: Selecting only one of the channels in the alpha. Select one of the channels in the channels palette. Then Ctrl+A to select all, then Ctrl+C to copy.
Option 2: Converting the RGB alpha into greyscale. Make sure the alpha image is the active document in Photoshop and go to Edit>Convert to Profile and choose sGrey from the drop down list and click OK. Then select the entire alpha image with Ctrl+A, then Ctrl+C to copy it.
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Comparing two renders - with/without a black background. Both renders would produce the same alpha map and the illumination on the sphere itself is exactly the same. The sphere has been rendered deliberately out of focus to show what happens at the edges of objects (which always fade into the background even when in perfect focus because of the antialiasing applied). 
The first sphere has a background which changes in brightness and the out of focus parts of the sphere mix with this changing background. After using the alpha to cut it out of the background, those areas with changing brightness remain. It is not possible to remove them using either "remove black matte" or "remove white matte".
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