Could you imagine the power that would be needed to process all thought in the human brain... now multiply that by 300+ million folks.
there is really not a whole lot of stuff going on in the human mind; a computer process does more exotic things than anything I see going on inside any human. the most complex thing we do is combine all these signals into what we perceive as reality. tell me when again the human can run a complex physics or mathematical simulation and basically instruct their own mind to do whatever they want it to in a programmable method. It can't do anything like this.
but for the purpose of mind reading, it's also not hard for the military to build custom stream compute processors to handle large amounts of information. I am thinking they could contract with IBM, NVidia or AMD for some exotic stream compute chip, or design their own without the limits of the consumer market. Basically they work with budgets of billions per computer network, far above the $1000 consumers spend on these devices. Sure sounds like they don't have the money for a computer network to do all this.
a small computer could easily track all the images and thoughts of a group of people going on in a small area. add then multiple of these systems to expand coverage. I know the police in town are rolling with this technology in their trunks, or at least the CIA and FBI dudes are.
ps. what if they could use your mind to figure out what they wanted. this is one of their techniques; the mind does all the work for them. it tells them where the signals are located that stores what they want, it tells them really easily what something is, or what knowledge you have or possess on something. they may use your own mind and thought to weed through information and reduce the amount of searching and processing they have to do. this is a brain computer interface, an extension in computer form of your mind. It literally grows with knowledge based on what you know and what your own expertise on something is. this information may be used to determine how you interpret the world, and this is used by the computer to understand what it's processing and dealing with. this reduces the work the computer has to do.