Because that stand is 3.5' long when collapsed. Kinda unwieldy in a crowded airplane cabin. The one I got is only 26" long. So that + an extension is better for airtravel.
A word of caution here. That stand is 2.5 pounds, 26 inches collapsed. That is pretty light. To me definitely not a viable weapon. Hey they let me fly with my 18" "laptop" that weights 12 pounds. Now if I wanted to do some damage and did not care about the machine, I could knock someone cold with that thing easily. For that matter a 722 is built like a tank and fits nicely in your hand - you could brain the entire flight crew, albeit one at a time with one of those bad boys.
But TSA does not exactly make sense, and although they are in principle a single organization, I have had radically different experiences at different airports.
It certainly is done - when I was flying up to the Philly Dead shows in May another guy on one of my flights had a stand probably similar in size that he brought on.
You may in fact be able to carry it on. But I would not just assume it is a given.
The thing about a 26 inch stand is it is a lot more fly worthy because you can put it in any number of more or less regular pieces of luggage if it does have to be checked. I'm sure you guys have seen these bags that have the pull out handle and rollers like a typical suitcase but are long and narrow - basically a duffle bag with the wheelies. That is usually what I fly with, and a 26'' stand will easily fit in one of those, as opposed to the big bulky stand I have which will not fit in much of anything other than a ski bag.