ducd, that's the point. By only giving you half the answers, they provide enough material to learn from and test yourself against. Instructor's manual has all of the answers, so its up to the teacher to provide them.

In my physics classes, we were usually only assigned the problems with the answer in the back. We were graded on solving the problem, not providing the answer. Being able to check your numerical result against a known-good one was invaluable.

"Lazy authors!"

But there's extra money to be made by selling the solutions manual!

There is another side to this. In upper level mathematics courses providing answers to all (or even most) of the questions actually does a disservice to the students. It teaches the important skill of deciding if you got the answer right which is probably more useful in research/the real world.