WotC has gone back and forth with the availability of the core manuals, and others, for AD&D since they re-released the PDFs after all the hullabaloo a few years ago when some people put the PDFs online for all to take without the copyright holders getting their cut. Many agree that WotC overreacted, but a year or two later, they relented.
When they re-released them, they made them available on a new site affiliated with OBS, D&D Classics. All the materials you buy at Drive Thru RPG, RPG Now, and D&D Classics all share a common library, and once you buy them, you can download them whenever you want, even if those titles are no longer for sale.
I like AD&D, it is my preferred, go-to RPG. I want to be able to get others interested in AD&D, or parts of it. Those involved in the OSR know about how great a resource the Dungeon Master’s Guide is. There are only so many hard bound copies available, so without a means to get a legal PDF, people do without, or break the law. While the DMG and the Monster Manual are not needed for players, it is difficult to have players that don’t have access to the Player’s Handbook.
OSRIC and other clones are close enough to AD&D that you can get by without the original rules, but there are differences to be aware of.
For in-person play, it is not an issue, as I have multiple copies of the Player’s Handbook. I also add to my collection by buying all the 1e PH’s I see when I visit my FLGS. With online play, one needs players with their own copy of the PH or OSRIC or pick another clone for the rules.
I understand that 5e is the current latest and greatest, but I would like to be able to run things in an online game and have legal access for my players. I’m not going to make available my copy of the Player’s Handbook, because it has my name and account number on it. Right now, 3e is available as PDFs, and the AD&D 1st Edition books other than the big three are available. Until the Player’s Handbook is available, I will make do with OSRIC.
I also would like to get Chainmail and the OD&D PDF’s. I’d buy the OD&D wood box reprints, if I could find one priced at the level that it came out. WotC’s website is not a commerce site, you can’t buy direct from them, at least not that. I’ll just have to make do with clones, like Delving Deeper.
EDIT – As Requested, here are the links to the OBS sites for obtaining manuals and modules. Just search for the one you want.
DrivethruRPG – http://www.drivethrurpg.com/index.php
RPGNow – http://www.rpgnow.com/index.php
D&D Classics – http://www.dndclassics.com/