None of these companies bought the actual SDK from Telechips. If you're putting a ~$150-200 tablet on the market, you need to keep the cost down.So, all of the work is outsourced to external companies, that they hired to program the firmware for them.Those external companies (Korea, India, sometimes China) have the cheap labor force (aka programmers) make firmware.And, those externals are the guys who sign the NDA's with Telechips so they can buy the SDK which isn't cheap (these costs are being saved by Augen for example.)The chance of these tiny companies violating the NDA for the buyer(Augen) is slim. I've seen NDA's and the penalties are very high..sometimes those documents are inside dev kit leaks that less scrupulous employees sometimes put out

Sure, GPL is legal and binding in de USA and Europe, but this doesn't mean that somebody in India, China or South Korea will follow it.. but there are off course examples enough on gpl-violations.org that should serve as a big fat warning to those who don't follow it