Ummm...
Because no one really cares.
And because the Mods don't have time to fix every one of my stupid errors (that would be a full time job in and of itself). The error is corrected within the thread.
Besides, Thomas Sowell made the same mistake in his column, so I'm in good company.
Here's Thomas Sowell, distinguished Professor of Economics, making the same mistake:
http://www.freerepublic.com/focus/f-news/1801064/posts