The TI-84 Plus C Silver Edition wasn't open to Apps either: the community factored the 512-bit App key on that calculator and released it for general use. Probably due to the outcome of the key-signing controversy
(ie, US law), TI Education did not choose to pursue the community over that factorization, and the community was able to release TI-84 Plus C Silver Edition Apps. On the newer TI-84 Plus CE, the App key is 2048 bits, which is many, many orders of magnitude harder to factor (many millenia on clusters of current hardware). I do
know some of the reasons stated for why Apps have not been made easier for the community to create on the TI-84 Plus CE, and Cemetech has been working with the relevant parties to come to some kind of agreement that will make both the community and TI Education happy. We'll update you when we have more information we can share.
As far as I heard they try to keep the key sexret because the calculators are allowed for exams like they are now. However, third party apps open new possibilities for cheating and so on, and thet calculators could get forbidden. That's inaccurate. Community Apps cannot do anything special that ASM programs cannot already do, in terms of potentially test-compromising functionality.