Deep, in all due respect, split hairs far too finely. When you phrase your sentence "Dr. Steven Jones found that a sample of structural steel from WTC7 had had certain characteristics...", you end up implying that Jones inspected the steel member directly, or inspected a component that was directly tied to components from the WTC. That's the logical conclusion to draw from the way you constructed your sentence, all protestations to the contrary nonwithstanding. If you recall his paper, no such linkage is established; the dust sample was merely presumed to be from the towers collapse. Due to the location of it's collection, it's an assumption of reasonably high probability, but it's still only an assumption. Your phraseology shouldn't have implied that the dust was directly tied to the steel when that's not established.
On top of that, Jones asserts that his findings indicate reactions at rather high temperatures that would presumably not be found in such a fire as was experienced on 9/11. As
Ryan Mackey has noted, that's not a given; iron microsphere creation can be catalyzed at lower temperatures than what he postulates.
Now, regarding fly ash: It is a far more probable hypothesis because it's genesis is known to have been involved in the constuction of the towers; such ash is a component of concrete manufacture. Whereas alternate hypotheses positing that the microshperes were caused by agents such as thermite lack supporting evidence, such as characteristic effects left on the steel members, lack of observed phenomena that can be attributed to thermite, lack of certain other elements being present (such as barium, if you're talking therm
ate) ascribed to thermite, etc. On the other hand, alternative hypotheses that are consistent with the known history of the towers - welding events, for example (I'm not satisfied with Jones's handwave dismissing welding, but I'm willing to accept that his criticisms of it may have a modicum of validity) are a far more reasonable source, because no deus-ex-machina explanation is required for them to be true. The point here is that it is reasonable to dismiss or assign a lower probability to fantastic explanations when such explanations require far more conditions, and far more unusual ones at that. That's how Occam's Razor works.
And regarding sulfur: Given the number of years the towers were standing and exposed to diesel fumes from vehicles, I'd be far more surprised if he
didn't find any sulfur. I know this is after the fact, but sulfur emissions are enough of a concern at Ground Zero that there's an initiative to reduce them (
http://www.cleanaircommunities.org/projects/wtc.html). Consider how much more exposure the towers had during the decades they were standing, and in times when such environmental concerns were merely beginning to develop. Add that source to the known contents - drywall, computer monitors (tube ones, not LCD flatpanels), etc. - and it's a wonder why the presence of sulfur is considered indicative of therm*te. Alternate sources are not only known, but are in abundance.
