I see what you seem to be seeing if I use the context menu. However, if I do the usual Cmd-C thingy, only the selected area is copied. To be precise:
- Select a rectangular area, Cmd-C, Cmd-N
- Result: A new document containing only the selected part of the PNG
Anything else would be really, really astonishing. PNGs can’t be protected against cropping. And Preview has been able to do that since ages.