Why not open source all the code? What does Google have to gain from closed sourcing even part of Google glass?
1. Google wants to monetize the novel features of Glass
2. Google is bound by licenses for the technology, and is unable to open source certain bits
3. They will open source everything eventually, but decided to rush this bit for publicity reasons 4. Google is required to release the kernel code under the GPL
The big pay-off for google is that to use glass you need to use google. If the devices were 'open' then you would not need google to use glass and they'd be in a cue-cat situation.
Also, the Ars Technica article does not discuss what I suspect is the most interesting part of Google Glass: the code they run on their servers.
There, it's more or less the same story: they release commodity lower layers, but keep the more interesting upper layers to themselves, for the same reason: money.