As a layman, even I could tell you why the need to be above the towers is a perfectly plausible condition....
...Because, you would need line of sight to target the buildings.
What exactly do you think the size of this weapon is, or how it is used? Without any of this info, which is pretty basic, you can't assume a DEW was used just because you don't like the way the collapse appeared.
Unless this DEW is the size of a pistol or something that some guy across the street could've pointed and shot at the towers, it'd have to be above the towers aiming down, because If this device was a large weapon at street level, it surely would have been noticed by someone.
Being that NYC is a bunch of towers of concrete/steel/what have you, If it was absolutely anywhere else but in close proximity to the towers with line of sight, ANY other structure in the DEWs its path would be dustified as well. Does this weapon have the ability to weave around other structures? If so, where is your evidence of this? Hence - line of sight is necessary.
A mediocre, sophomoric attempt, but I believe I got my point across.
Does that compute?