While I too wouldn't shell out that much for a PDF, I don't really think that the majority of us are the intended demographic for that product.
First, I can't imagine that offering a PDF version of an already existing book costs very much, so it's worth a few extra dollars to extend your product to an audience that likely it wouldn't reach. Second, that audience is likely those overseas.
Imagine that you live somewhere far away from the US. And you'd like that book - but the print version is 50 bucks - and then you'll have to ship it. International shipping on a large book can't be cheap, and if you want the book sometime in the same lunar cycle, it can quickly cost you twice or more the original price of the book!
So the question becomes not "Who would pay $50 for a PDF when you can get the book for $50?" but "Who would pay $100 for a book when you can get the PDF for $50?" And for the small incremental cost of the PDF, you've picked up a handful of sales you'd never have made.
And you turned a tidy profit on those few sales too!