The gemara says that the machlokes between Beis Shamai and Beis Hillel regarding the shiur for chometz whether it is a koseves or kezayis, is only for the issur of בל יראה ובל ימצא but for the issur of achila all agree that the shiur is kezayis. Yet, when explaining the opinion of Beis Hillel the gemara says that the Torah needs to write both chometz and se'or. Had the Torah only wrote that chometz is assur, one would have said that only chometz is assur since it is eatable but se'or which is not eatable is mutar, therefore the Torah also had to write se'or. This doesn't seem to make sense. Why does it matter that chometz is eatable and se'ora is uneatable, since the discussion is not about the issur to eat chometz, rather about the issur of בל יראה ובל ימצא? Why would the fact that chometz is eatable be a factor in explaining why there should be בל יראה ובל ימצא?
Usually the role of the Rabbonon is to make a gezeira on something which is assur m'doraysa, but there are a few places where the Torah itself makes a gezeira to prevent a more severe Torah violation. An example of this is the issur yichud which is d'oraysa as the gemara says in kiddushin and avoda zara, yet it is clearly to prevent relations with arayos. The Ran suggests at the beginning of Pesachim that the issur of בל יראה ובל ימצא is also the same type of issur. It is a "gezeira" d'oraysa, to prevent the eating of chometz. The Ran explains that the reason that the Rabbonon were so machmir regarding the issur of chometz to demand both bi'ur and bittul is because they realized that if the Torah was so machmir to prevent the eating of chometz (by demanding that it be destroyed) they should also make gezeiros to prevent it. Rav Yosef Engel in one of his seforim has a list of issurei torah that he considers to be of this genre. Based on this approach it is not understandable why the gemara considers the issur of בל יראה ובל ימצא to be more likely to apply to chometz which is eatable rather than to se'or which is not eatable.