They aren't trying to hide it as I was able to find Chapter 540 in total, although your link didnt work. So which sub-chapter are you alleging that says evidence for the defense is not to be heard by the Grand Jury?
But fwiw I found this which seems to answer the Prosecutors ability to present evidence:
http://www.moga.mo.gov/mostatutes/stathtml/54000001401.html
Emphasis mine. I don't see any limitations on what information/evidence the Prosecuting attorney is able to give to the Grand Jury there, do you?
Page is up and fine, although you cant use your link to find it. However, I think I just quoted you the correct part of Chapter 540 that entails scope of duties of the prosecutor.
Probably because their testimony was 'relative' to the matter before them. As per C540.140.
Whatever you had read earlier, I think you may have misread it or mis-interpreted it.