1. Last time a 'city-killer' struck was 100 years ago, it just happened to be in an empty place.
2. This isn't a panicked response -- you can't build a satellite in a week, and this has been on the books for years. The Russian meteor is a nice reminder, but this has been a focus of many for decades.
3. My background is in astrodynamics and spacecraft design. I can do a lot better work on protecting against asteroid impacts than I could focusing on nuclear weapons. The best man from my wedding is a nuclear engineer, who does in fact work on non-proliferation. Beyond the two problems you mention, we (humanity) face a huge number of troubles, as individuals, nations, and as a whole. We don't tackle them one at a time though, and we shouldn't -- assuming we should is basically the fallacy of 'The Mythical Man-Month' writ large. The amount of time and money spent on detecting asteroid threats (and considering options to disrupt a threat) is appropriately minuscule next to that spent on other problems.
4. Developing satellites to better detect small asteroids is far different from fear-mongering against nuclear power or for anti-terrorism policies, because you have to consider side effects. Nuclear power is the most reliable way to provide non-carbon-emitting 'renewable' base power, and limiting it based on fear is thus disastrous, while reasonable fear leads us to build safer plants. Anti-terrorism policies at their core aren't bad, its only that they tend to have the nasty side effects of damaging civil liberties -- radiation detectors at ports (for instance) are fairly inexpensive and unobtrusive and thus a reasonable response to a real but overblown threat. The side effects of launching satellites to help detect city-killer sized asteroids (which is also inexpensive in the grand scheme of things) are to learn more about small bodies and the evolution of the solar system, and to develop and improve spaceflight technology. This is hardly the same kind of bargain as the fear-mongering examples.