Alrighty, looked into this today while working a new feature as part of a request, and made a new discovery. It's not really a bug, just an inconsistency, because you can already control this behavior via the options menu! It's under the Default Image section, where you can select "Background." As per the context help there, it applies to new images (including resizing) as well as post-cut areas.
I've done two things here: set the default image (auto-created on open to fill the canvas) to also obey that setting, and set the default for that option to use 255,0,255 rather than 0,0,0, so that its behavior is more predictable for newer users.
So what's the new feature I was working on? It's actually related to transparency, and made exploring and addressing this issue much easier
There is a new toggle ('n') to enable "Explicit Transparency" mode, which displays all transparent cells for the current layer (all other layers are hidden).
It'll be in the next release, but
here's a patch to get this feature now(edit: feature now included in all versions as of v1.50). Note that the patch also overwrites the skins.xt file, because some new entries were required there. There you can modify the colors used for the transparent area, and there are actually two sets as you can see in the demo above, a dark one and a light one (toggle between the two with Shift-Alt-n).
Also, you'll almost certainly want to open your options menu and change the Default Image background to 255,0,255 so you can actually see the effects of this mode