Yes it is application specific. Until today had never seen an 8bit representation of the alpha channel in a png as i do with say a targa. I did a bit of reading and photoshop is specifically hiding the alpha channel in png’s
From the adobe help desk., Transparency and alpha channels and png
PNG does not support arbitrary alpha channels like other formats such as TIFF. PNG specifies that the fourth channel in a file is transparency, and only transparency. When you open a PNG file with transparency in Photoshop, it is considered a single layer image. It is not a flat background image. Alpha channels can contain anything, while transparency is a specific channel relationship. You can have multiple alpha channels per document, but only one transparency channel. Photoshop handles transparency and alpha channels separately.