Compared to the PC world, calling the iMac an "entry level" machine is ridiculous, but even in the closed environment of Apple's own products, it doesn't stand up. The Mac Mini is entry level. The iMac isn't. There's no excusing this as "entry level."
6 bit panels are very common across the whole industry and the main reason (apart from cost) is the faster response times of 6 bit panels.
The panels do actually display millions of colours but use dithering to achieve this, so Apple (or any other manufacturer) aren't actually lying
Edit: If you want to see what type of monitor you have, this site may help: http://www.lagom.nl/lcd-test
I found the most revealing test was the viewing angle one, which remained rock solid on my Cinema Display (as you'd expect with it being S-IPS), but revealed my MBA display to be a TN one: http://www.lagom.nl/lcd-test/viewing_angle.php