They did. By lowering the price. I used to have 2 PC's running pirated copies of windows. Not anymore, they are both running Windows 8 I purchased at a reasonable price of 39.99.
I do have Windows Vista OEM licenses which give me the right to upgrade to 8, just no windows 7 licenses which i used to run 'illegally'.
But I believe Microsoft knows what they are doing, cause the upgrade assistance does not check if you are running a legit version. It either means they were too trusting that users will be running legit versions before upgrading or it means that they deliberately left this hole in order to get a revenue from users who would otherwise just install a pirated version of their new OS.