Yes, this was on purpose, the idea being to keep the work you put into the mask, while resetting the color curves. There is a Master Reset button available in the customize dialog that you can add to the toolbar. It will do a full reset, including resetting the mask and it's channel.
I know there were discussions on this over the last week, so is the current behaviour the final one? I spent about 15mins last night trying the latest CM3 beta, and almost immediately hit this Reset issue. I wanted to get back to the starting point, but the Reset button on the curves pane no longer does that. Since I had experimental curves in 3 colourspaces I had to select space, click to reset, select next space, click to reset, click last space, click to reset. Then think about the masks. I found it easier to hit Cancel and restart CM3.
It is, IMHO, unfriendly, hard to use, and totally breaks the principle of least surprise. If I see a Reset button in a Photoshop context, I expect it to reset everything, not a subset of things that I might or might not expect or be able to predict without thinking it through. I wouldn't trust that reset button anymore. Its function is no longer intuative.
The PS way of doing things, as I understand it, is to have the Cancel button become the Reset All button if the Alt key is held down. Hiding the most powerful reset behind a metakey press like this makes sense, so if that were possible the current reset button could be left alone apart from relabeling it "Reset RGB Curves", "Reset Lab Curves" or whatever the current colourspace is.
I appreciate it's a bit late in the CM3 cycle to start making big changes, but I think all this Reset business needs working out for the first point release.