I put it there because it would be awesome if it were there so I don't get poisoned and die - that is my actual reason. If you need backup reasons, I've stated them. The Shield of Faith is a piece of defensive equipment. I don't always activate it right away because it has a limited length of use, so who knows, maybe there was one before there was that attack on Teth, maybe it was built since what with the advent of war. Does it matter just how soon it's been?
I've had Teth for at least a few months of real time, so I guess that's long enough to intergrate some of our stuff. The resources needed to maintain it are hardly costly - the jewels are mined from Azguard, and the rest is just rocks with symbolic carvings. The operators aren't Jed-quality force users, they're just above-average sensitives who feel the Force through some minor way, like tarot-card reading or the muse of painting or some such outlet.
In fact, this system has a useful peacetime application too - for the minimal cost of their necessities, these people can serve somewhat as state artisans, weaving the power and philosophy of the Force subconsciously and subtly into commissioned paintings and works of art. I think I've hit upon a really cool idea with this, something very interesting that in peace I think I'll expore more fully, but for the time being I would say the resources to set up such a network would be minimal, to my way of thinking.
As such, the defence of the planet relies on one of these Force Technicians, if you will, sensing something through their chosen muse that hints at coming disaster. For this character, he let the Force paint his picture, and it showed death raining from the sky, so he raised the shield.
I don't know if Teth's population is mostly droids, I wrote the takeover assuming an Earthlike population, as all I could find on it was that it was a human planet and such. I tried asking people like Jan and Joren about it, and what I came up with is what is in the takeover (minus the silly). Admittedly, I never specified how much 'courage' would be required to power a shield. More than one, but more than one billion? I dunno.