Well I think the confusion is not because modern law does not make distinctions between hadd and siyasa etc; the confusion is because of traditions. Under Islam hadd is a very serious offense and therefore the defendant's (or accused) invocation of qadhf serves as a strong shield (the requirements of evidence are clearly described in the sources of shariah), however taazir or siyasa offenses are judged by circumstantial evidence made available to qadi or ruler and thus the defensive shield of qadhf is made weaker. This has some consequence such as giving rise to prosecutions under false accusations of rape, e.g. a woman had consensual sexual intercourse (or relations) with a man and later accuse him of rape (for reasons better known to her ). This seems to be the source of confusion as placing rape under hadd gives advantage to the rapist and vice versa.

In other words what I meant to say was the source of confusion is more political than legal, since the legal complications can be overcome if rape considered as an act of sexual violence as you said. As in the words of Professor A. K. Niazi:

Those who claim that this off ence has been created through the implication of the text (dalatal al-nass) should realize that the attribute of sex has been given priority over bodily harm. This goes against the maqasid al-shari‘ah [the objectives of shariʿah]. Interpretation is undertaken according to the purposes of the shari‘ah (maqasid), and the maqasid maintain that priority must always be accorded to bodily harm over matters of sex. Accordingly, rape, which is an attack on the physical person of the victim (including mental agony),
cannot be included in the offence of zina. It is an offence that falls in the category of hifz ‘ala ’nafs

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