a) Yes, it's a beta, but why not have everyone test out your product rather than limiting it to Windows users? So, they complete a Windows only beta, then start a Mac beta, then a *nix beta? My point is simply that there are a wide variety of web technologies available to companies. It frustrates me when companies choose to alienate some of their readers by choosing proprietary solutions, especially when those companies are as large as the NY Times.
b) Yes, Macs are a smaller share of the market, but they are a share of the market nonetheless. There are also other OSes, and I'm sure some *nix nerds would like this service as well.
For the mac users
Yeah, this is great if you have a new Intel Mac, a Windows license or the money to poney up for Parallels. Thankfully, I have all of the above, but I would put money down that only a small share of the Mac-user world meets the prerequisites.