Mac users are limited to PDB downloads because the official B&N Reader (note, not a NOOK app like Windows users now have) crashes and burns whenever DRM-ed ePubs are involved, often on a random selection of ePubs with seemingly no particular pattern to the failures. Though it works quite well with non-DRM ePubs.
This prompted quite a spate of complaints on B&N's discussions boards, to the point where B&N, who had just switched over to All ePub! All the Time, finally reverted the Mac users back to head off those complaints.
And now they get new ones about how Mac users want to download ePubs, so they can't win, unless they put up a "choose your format" button on the My Library page, at which point someone will no doubt complain that's too difficult a concept and why can't B&N make downloading a simple 1-click process?!?!?
Incidentally, the release of NookStudy does not solve anything, because it is limited to 10.6 Snow Leopard Intel users and thus a large chunk of people with non-Intel and 10.5 or lower Macs are left out. The non-Intel Mac people, incidentally, had already been shut out by the most recent "upgrade" to the B&N Reader software, which dropped PPC support.
NookStudy also doesn't read one's old PDBs, which is something you'd think it would support, considering that a not-insignificant percentage of the "NookBook" listings say something along the line of "not compatible with Android client" or whatever, which is the secret code phrase for PDB-only, just as "not compatible with Mac or Blackberry clients" means ePub only.
Personally, I'm perfectly fine with the fact that they still offer PDB downloads to Mac users and that it's pretty easy to also get the ePubs from this platform if you're motivated enough.
The even older B&N eReader app which they used to have on the website and which is still available via the eReader.com website is the only really convenient way I can read my B&N "purchases" on my ancient 10.4 PPC PowerBook, short of stripping and loading them all up via Firefox's ePubReader add-on, or the awfulness that is Stanza's desktop app. But I believe I mentioned convenience, so strike that last part.
So now you know. And knowing is half the battle, as GI Joe would put it.