Why would they try to shoot them down? for what purpose?
An ufo crashing in the middle of Arizona city would be kinda hard to cover up...
An aggressive act towards the ufos, wouldn't that upset them? UFO REVENGE 
Seriously... if (successfully) attacked, wouldn't they 'shoot back' ?
anything that has tc36-40 catagories in the us military special units has no obidence to the constitution and can take over the country in case of severe catasrphy ,,it is a last resort after all the other resorts are used and they would use every resort before hand in case of severe catastophy,,
tc32 and under is president based and it wil be executed to the max before utilizing tc36-40
the subject of ufo as you call them is catagorized tc40,,it is the most important secret the united states of america and many other goverments old in their own regions,,there is nothing that is more important in a country that in the know than to keep this at us will since us is the only super power as tc 40 level or the equivelent level of those countries
tc36-40 as i said has no constitution obeidence,,they shoot to kill ,,they tortue to get information and no im not talking about water bullshit from iraq,,they do whatever they want inorder to get what they want ,,if it means they need to kill you ..you will be dead if they can know who you are ...if not they will make sure you look and sound like a lunatic in the mainstream society,,this sciety is built in specific structure and believes that must not be shaken by anything as of 2007 for reasons im aware of and some that im not and probably dont want to be aware of
so,,
they shoot them down because thats what tc36-40 does when they have them in sight,,they try to and never suceed ,,it is always failure from thevehicle that cause it to crush if crushes due to atmosphereric conditions
a ufo didnt crush in the middle of arizona but if it did it would be as easy to cover up as it was to cover up the other crashes such as 1947 roswel,,when you have no obidence by constitution you can do whatever you want
agression acts twards extra terestrial until now caused no harm ,,they do take people but it has nothing to do with reaction to what we try to do to them as in shooting them down,,we were never sucessful in shooting them down ,,i never was in one of those vehicles i dont know their way of protecting it ,,i assume the heat of 1997 incident has something to do with it but i really dont know ,,it has something to do with the graviational field/shield created by their anti gravity propultion ,,but i really dont know specifics on that aspect