Background layer default opening vanishing

Hi, when you open, let's say a PNG file, it creates a default locked Background layer, fine, now edit it, turn it into Layer 0 for example, and save, again as PNG, ...the next time you open that file, you'll get your Layer 0 and not a Background layer, I definitely need to get rid off that behavior, I need that Background Layer to be there! PNG isn't a PSD or TIFF, why is Photoshop saving Layer infos? and where? in the PNG? in a Cache ? I tried a bit of things to retrieve that "Background layer" when opening but PS CS5 still stores that info somewhere...