As a co-author of the books I want to respond to this because I've heard this criticism a few times and there are definite reasons for the layout of our books that aren't as much a conspiracy theory as you suggest (our books didn't kill Kennedy, either...).
1) The one-problem-per-page setup is a fantastic way to study. Students can show their work on that same page, dog-ear or bookmark the problems that they'd like to discuss, and bring them back to an instructor with a full blueprint of their thought process. As an instructor, this allows me to review a student's work quickly during breaks in class and make appropriate recommendations.
2) The one-problem-per-page setup is also ideal for a classroom situation in that it allows for open space in the workbooks so that an instructor can suggest tangent rephrasings of those problems and create discussion and follow-up on that problem. As a teacher, I don't really care if my students get the problem on that page right; I want them to understand that question thoroughly by the time we're done. So I often take one problem as written and then offer 2-3 ways that the GMAT's authors could tweak that theme so that as a class we can construct the same concept from multiple angles to fully understand it. That white space gives students a place to make those notes, copy down the rephrasings of problems, and have a single reference page to review what we discussed.
3) The note-taking space on each page is quite convenient for young professionals who can simply grab that evening's workbook from a shelf, take it to class, and have ample space to take notes, work through calculations, etc. without needing to bring a separate notebook or loose-leaf paper. Similarly, that blank space on homework problem pages makes it convenient to work on GMAT problems on an airplane or in other confined spaces where it is less convenient to juggle both a textbook and a notebook.
Our page layout and design was constructed with pedagogical effectiveness first and foremost in our minds and student convenience a close second. And if you read the questions within the books you should be able to infer that we do care quite a bit about the environment; the subject matter of many example questions betrays some insight into our priorities and pet issues. You'll see quite a few verbal questions that deal with education, the environment, health and fitness, etc. But as an educator, I'll never see a book as a "waste of paper" - my concern lies much more in the mountains of paper packaging and advertising that accumulates in landfills every day. Look at the kiosks around any given college campus and you'll see that they're replenished just about weekly with a fresh blanket of Kaplan and Princeton Review flyers. As a test prep company, Veritas Prep contributes a disproportionately small amount to the killing of trees worldwide. And for the paper that we do use, I'm pretty happy with the return-on-investment that it provides our students.