Right, but the real role of ANY weapon is to attack. I don't think that fact is lost on anyone.
However, if you wanted, you could call both of them forum posting devices and be completely correct. Still, that isn't the point. The fact that makes computers and cell phones different from missiles in this case is that computers and cell phones each have many different functions. Computers can post on forums but can do a plethora of other things, as I am sure you are aware. Cell phones can also post on forums, but can make calls and do their own collection of tasks. Each of these machines has a very different purpose for being.
With missiles, on the other hand, you have one purpose - to destroy. Whether it is going Mach 0.8 or Mach 2.8, a cruise missile is moving a relatively large payload from point A to point B through the atmosphere to hit a relatively slow or stationary target. The only difference is how fast it is moving. The payload size and delivery method relative to other missiles is what makes it a cruise missile.