Hello, thanks for checking out my game and for your comment.
First of all, the elemental imprisonment/consumption is only an optional rule/suggestion, not a mandatory rule. If you don't like it or if it doesn’t make sense to you, you certainly don’t need to use it (and you can do this with any rule, of course).
You make a good point, and that also makes sense. However, from my perspective, I think that when you try to use magic and bend reality, you gain corruption because you are drawing from a power that is unknown or that you can’t fully control. If you succeed in consuming an elemental, you absorb that raw magical power and internalize it, thus reducing your corruption. Another way of thinking is that when you acquire the raw power of an elemental, you can channel that energy to cast more spells, thus lowering your corruption level (since you will be further from your maximum corruption threshold).