A well framed reply, and somewhat more considered than the 'OMG CAMERAZ' effort to which I initially responded.
You argue that the only perfectly safe computer is one with no network connections, and no hope of physical access. This is true. It does not, however prevent a sysadmin from doing everything in his power to mitigate risk to the items for which he bears responsibility; he will keep the things patched, employ a decent firewall, and so forth. It may not be perfectly secure, but it's a whole lot better than shrugging one's shoulders and concluding that because we can't prevent *all* incursions, it's futile to attempt prevention of any.
Would I have spent money on these cameras? No, probably not. Do they offend my sense of being a free man in a free country? Not really. If I'm out in public, I expect to be seen, and I don't particularly care who's doing the watching. I would be horrified if they were routinely bugging private residences of random citizens, but that's not the issue.
You question how many deaths have been prevented by the work of the intelligence agencies. I don't know. I suspect you don't either. It's impossible to say; what is concrete, and demonstrable, is that they've interrupted operations intended to cause harm, and retrieved materials capable of doing exactly that. Lacking a crystal ball, I cannot say for sure whether these planned attacks would have been successful, and I'm profoundly grateful that no innocent people were made to find out.
Lastly, I find your view of those in power depressing. I don't doubt that there are some as you describe, motivated by power alone; I assume a fair number are motivated by cash, and the desire to line their pockets. I do know, personally, one MP who's in it for good, altruistic reasons -- and I struggle to think he's the only one. It's a mixed bag, certainly - but the common factor is that none of them have anything to gain by alienating subsections of the population for no perceptible reason.
We pay the security services to do a job, and until they blatantly overstep the line, I'm happy for them to get on with it. I'm not privy to the information they have, I have no knowledge of the hints, leads, and clues which decide their strategy. I understand that if this information was made public, so the population could judge the merits of their actions from a position of full disclosure, their jobs would be made impossible, and there would be no point in employing them at all.
100% prevention would be nice - but I'll settle for mitigation. If you have failed to bury your computer, and yet use a firewall (etc), you are already doing the same. It's not a bad thing.