They'd get $4 a pop.
You're essentially asking "Ok, so WotC doesn't have to put in any effort whatsoever, and people pay them. But what does WotC get out of it?"
There's a reason why I said the last line of my post:
Sure, you might say "Well, they could have just kept the status quo" but I don't think it was enough to entice them.
Why did I say this? Because I feel it's quite possible for Wizards of the Coast to not be gaining anything meaningful from the files being available. This could include profit. They may simply not have enough customers of those books for them to overcome the costs of having the files available. Which are not zero.
There's no reason to assume that they would be getting four bucks of pure profit from sales of this material. At the least, the vendor would take a cut to pay for its operations(servers only seem free, they really aren't), and I do think Wizards would have to do some accounting on their end. How much would they be getting from each sale? Well, as far as I know, we don't know, but heck, let's handwave it as four bucks. Now how many sales would they have? How many would it take before WoTC was impressed with the amount of money coming in? Believe it or not, there is such a thing as not enough profit for a corporation. Want to handwave some figure? Let's try 1,000. For me, 4,000 bucks in money would be quite a lot, by a year, or a month. How much is that to Wizards? Well, they have sales revenues over 100 million, so not so much I think.
So yeah, I'm left still wondering, what does Wizards really get out of it? What entices them to keep the sales available?
This is something they'd have to answer, though, as I doubt any of us have the insider picture to say. Well, except for the WoTC employees who are probably under orders not to reply on these kinds of things. Corporate inscrutability and all that. But perhaps with their future plans for digital publishing, we'll find out. Or maybe their lawsuit will feature some information, depending on what the defendants lawyers do. I do recall some mention of RIAA figures coming out in a music-sharing lawsuit.