Well, I'm gonna be all pedantic and quibblish.
I reject entirely the idea that something cannot be scientific "by definition". As I have said many times, the nature of science is determined by the content of science. If a genuinely successful and useful theory fails to meet some defining criteria of what is supposed to constitute an acceptable scientific theory, then the criteria will be changed and the theory retained. This has happened over and over again in the history of science.
The only reason that creationism should not be taught is that it is not, in fact, part of the content of science (as can be determined objectively by consulting the professional literature). It may be interesting to opine about whether creationism could be scientific, or whether that is impossible for some reason or another, but this is ultimately superfluous. There either is, or there is not, some creationistic theory that "works," that is useful to working scientist pursuing their research projects.
At present there is not such a theory. I can't imagine that there ever will be such a theory. But if I'm wrong, and some genuinely successful creationistic theory should emerge, and be actually (not just as a pretence) utilized and implicated in ongoing research by working scientists, then fine. And if that should ever happen, then the theory WILL be taught in the public schools, simply because it will in fact be a part of science. That's how this all works.
It takes the scientific method and upends it...taking observations and forcing them to fit the theory.
Yes. Granted that this is how it works out, so far, as a sociological fact. For instance every creationist organization that I know of has a "statement of faith," and these invariably commit members a priori to at least some scientific (as well as theological) conclusions. However should some genuinely successful creationistic theory emerge (however unlikely that may be) then it would not require such twisted and ideologically narrow support.
The theory itself is never questioned or modified.
This just isn't true. First of all there is no "theory" of creationism. At best there are scenarios and narratives on the one hand, or vacuous sub-theoretical generalizations on the other; and they are multiple. Different flavors of creationism often differ from each other as dramatically as they each differ from mainstream scientific accounts.
Secondly, modifications do occur. It may not always be for the right reasons. For instance it may have to do with a balancing act between "how much bible can we work in" versus "how much conventional science do we have to jettison," but changes do occur. For instance it used to be dogma at the Institute for Creation Research that "entropy" began with the fall of man (after Adam ate the apple). ICR president Henry Morris insisted on this idea, but other creationists said this was nonsense, and the idea was quietly dropped when Morris retired. The "vapor canopy" theory also used to be all the rage among YECs, but it too has fallen into disrepute among most. (This is mainly due, I think, to the problem with the ocean boiling heat that would be released when the vapor becomes liquid. For whatever reason, creationists don't want to invoke a miracle here.)