Yep... you're wrong. But not that you
do want a proprietary, DRM-laden application... but you're wrong in that they're forgoing the revenue.
They don't want the money from the sales of PH PDFs. Rather, they want the revenue that will come from Dungeonscape. And I would presume that they've determined that the money they will earn through Dungeonscape will be much more than the money they would have earned from PDFs... enough so that they don't want to eat into the profits that Dungeonscape will generate.
If you took the group of players who
would purchase a PH PDF if it was available and then told them they could get the PH electronically through Dungeonscape... I'm pretty sure they've determined that enough of those people will choose to go over to DS and pay the subscription fee for it (since it's their only option with no PDF coming) such that it more than makes up for the money they would have earned just by selling a single PDF to each of the people in the original group.
If 20 people would make a one-time payment of $15 for a PDF for a total of $300... but 10 of those people would be willing to make a monthly payment of $5 for Dungeonscape for a total of $50 a month... the money equalizes after six months, and the rest of the year is gravy.
You might not be someone who will be a part of that group of 10, which I'm sure makes WotC slightly disappointed (since they'd love everyone to use it.) But the $15 you would have spent on the PDF will get covered more than easily by the others of you who will.