Because I do think it matters where alternate material comes from. My home campaign uses materials from many 3rd-party sources that wouldn't be available if there was no OGL. Things like From Stone to Steel and The Complete Guide to the Fey. I like a grittier game with a more folkloric feel that what WotC seems interested in producing. Therefore, cutting the OGL is removing a major source of inspiration for me. For my money, the Tome of Horrors books are more useful than the later WotC monster books. Removing the OGL would remove these sorts of products, too. Frost and Fur is, IMHO, a better product than Frostburn.
If I buy into a new edition, I want to be able to buy adventures as well. I might not run them, but I enjoy gleaning ideas from them. I recently purchased a new WotC module, and I absolutely hate the format. The module breaks the encounters into areas and tactical encounters, and essentially wants you to flip the pages back & forth to use the sections as needed. Horrible. They assume the use of a map for every combat encounter.....and the tactical parts read like you're playing DDM instead of D&D. Blech. Nice map, though.
So, without the OGL, who do I go to for adventures? Right now, my main choices are Dungeon, Necromancer Games, and Goodman Games. Dungeon will soon be gone, and without the OGL, neither Necromancer or Goodman are likely to support a new edition.
When 3.X came out, WotC got my money mainly through the Core Books and the other early releases. When 3.5 came out, I saw no incentive whatsoever to switch over; you can run 3.5 adventures perfectly well using the 3.0 ruleset. I don't like the weapon size rules (Monte Cook's AE does it better), the minis-centric combat system, or the changes to cover & concealment from 3.0. Now, I'm playing a hybrid, using the rules from each edition that I like, plus house rules (including many 3rd-party sourcebook rules). I like minis, and WotC certainly got some cash off me there, but I don't care to have them mandated by the rules....and certainly not by adventure modules!
WotC seems utterly uninterested in producing the type of content that I enjoy, and a non-OGL game means that no one will produce that content. So, what is my incentive to switch to 4e?
Finally, in a free market economy, the only way that a consumer can show his displeasure is by not buying. I think that the OGL is the best thing that happened to D&D since Gygax took paper to pen. I think it is the polar opposite to the policies that T$R took on to counter personal D&D websites back in the day. I think that, while the brand name belongs to WotC, the game itself is larger than that, and is best when the creative efforts of many, many people hear voice....even that initial glut of d20 products contained gems. Even some of the stinkers had a few really good ideas in them.
The essence of what makes D&D worth playing is that we view it as our collective sandbox. That disappears without the OGL.
Therefore, if 4e is non-OGL, I will not buy it.