Alpha channels are one of PS's most important tools, but they are something different than how alpha channels are used in other apps. For one, there can be an infinite number of them (so the one that would provide the 'mask' should be appointed differently). Actually, the 'transparancy' we accept as normal in a photoshop layer is a sort of 'hidden' alpha channel. When you click a layer, the transparancy channel suddenly appears. Alpha channels can contain information about selections (save selection generates an alpha channel), but also about additional color channels when you work for print. A 100% black alpha channel contains no information. If you want a black background, that should be a layer, the transparancy of whatever is on top is 'hidden' alpha information. So maybe that is why it is changed to a white channel, which contains no information too. There is not much of a difference between 'select nothing' or 'select all'. They are both taken care for in other areas of the program... If you open an alpha mapped image from 3DS Max in Photoshop, you have to go to the alpha tab, and make a selection of the 'maks' alpha channel (option click on it). My 3D app can generate, like, 30 alpha channels, all with different information like depth. A .tif file wil use the first alpha channel as a mask in a lot of applications, but Photoshop gives you all the freedom to use, mix and als filter alphachannels. I could not create nice grunge type without them. So maybe it is just a slight difference between the meaning of the term 'alpha channel' in different programs.