Alleyways To Torah

If I had to guess, which is clearly what I am doing, I would say that the גזירות are focused on things that one could learn on שבת itself or, at least, by the next day. In other words, the way I always understood it, though it's probably faulty, is that one might come to carry it not out of carelessness but out of motivation for עבודת ה, i.e., one wants to be able to perform a מצוה properly. One could theoretically learn parts of the מגילה, learn to blow שופר, etc., but it seems much more difficult to learn how to do מילה in a single day and be confident enough in one's proficiency to acutally perform something as important and central as ברית מילה.
גמר חתימה טובה

Ari - a פטור קרבן doesn't necessarily indicate that there was no מעשה עבירה (the question even exists by אונס). To respond to your second point: that reasoning is a bit circular. You are saying that the מוהל is going to perform a מצוה דאורייתא because there was no גזירה דרבנן, but לו יצוייר there was no גזירה דרבנן on blowing שופר it would be exactly equivalent to מילה in this regard. The whole דיון here is precisely about the איסור דרבנן, there presumably would be a חיוב דאורייתאwithout it (and some question the remaining existence of a קיום דאורייתא if one violated the גזירה דרבנן). Thus, לענ"ד the person carrying the שופר was also on his way to do a מצוה דאורייתא before the גזירה was enacted (just like by מילה), and Rav Ally is asking why they were only גוזר on שופר and not on מילה. One can, however, possibly distinguish between a person going to perform a מילה and a person going only to learn how to blow שופר, but not to actually blow for the רבים at that moment (or even on that day).
פתק טוב.