It would be if the problem was worded as so:

I have a piece of 8.5 x 11 paper and a set of rectangles to cut. Tell me either one:

• How many rectangles can I fit on the paper?
• What is the fewest number of cuts for the largest number of rectangles?

The problem, as stated, is different. "Given a set of rectangles, what is the smallest piece of paper that will encompass all the rectangles with the smallest amount of waste?" The reason this isn't a Knapsack is because the overall paper-size isn't constricted. (At least, the OP didn't restrict it in the statement.)