I'm sure it pisses off $5000/logo designers as much as iStockphoto pisses off people that used to make a good living from stock photography, but welcome to the 21st century.
Should you ever get caught with a logo using stolen works, you're setting yourself up for some really hefty fees regardless of where you obtained the work. This isn't conjecture, it's happened before:
http://www.thelogofactory.com/logo_blog/index.php/anti-spec-...
As the article hints, it's a problem of economics. With spec work, the artist only gets paid on a few works out of many. To make many works and still earn a living wage, the artist must make each work with a fraction of the resources usually allocated to a professional work -- which on average can't compete with stolen works produced with more resources.
If spec work paid more, real artists might be able to compete -- but the only reason people want spec work is because it is cheaper... in the short term at least.