The Bnei Yissoschar explains the reasoning for this: A kal vachomer is based upon logic. One might say that the reason this halacha (derived through a kal vachomer) is correct is because it is understandable to me; it makes sense. The Torah therefore goes out of its way to write it explicitly in order to teach us that the halacha is correct because the Torah said so; regardless of whether it is understood or not.
The Ra”n in Nedarim (3a) notes that this concept is applicable by a hekesh (when the halachos from one topic are derived from another one) as well. The Gemora in Bava Metzia (61a) states that it also applies to a gezeirah shavah (one of the thirteen principles of Biblical hermeneutics; it links two similar words from dissimilar verses in the Torah).
According to the explanation of the Bnei Yissoschar, we could say that the concept should only apply to a kal vachomer, for that is based upon logic. The Torah would not find it necessary to state explicitly a halacha which is derived through a hekesh or gezeirah shavah, for they are not based upon logic at all, and it would be superfluous to write it.
The Yad Malachei writes that if the Torah does explicitly write a halacha which was derived through one of the thirteen principles of Biblical hermeneutics, we must treat it more stringently than an ordinary halacha. This is comparable to a Rabbinical prohibition, which has a slight support from something written in the Torah. Tosfos in Eruvin (31b) rules that such a prohibition is stricter than an ordinary one, which does not have any Scriptural support.