Decent quality? maybe not so much... but you are right that you can buy them much cheaper. Sadly, you don't know whether the parts in those shenzen tablets are legit, and where they came from. TI has a certain quality level to adhere to, so for the firm to get down as low as $70 is really pretty impressive.
I think eTAB's advantage comes with the customization, design support and quality. eTAB solution offers customer the opportunity to customize their solution for specific market needs and system requirements; design also comes with hardware/ software design files including schematics, layout, source code drivers; also not to mention TI has great reputation in embedded solutions market for decades. It would be ideal for enterprise tablet applications.
The Shanzhai mentality is "cheap" and performance cost ratio. They just focus on some performance parameter and even presenting some fake numbers. There are still many link claiming A10 running @ 1.5Ghz, which actually run at less than 1GHz.
They don't really care about user experience and reliability.
There is a definite need for OEM, function specific tablets. TI appears to see this need and can provide longevity of this product with its low cost, high performance Sitara line. That's important for an OEM, compared to using an off-the-shelf, latest and greatest. In addition providing a Linux tablet fits well with OEM, custom design. This will also leverage other other alternatives to application design, C++, Qt, etc, outside of Android and iOS.