I'm not sure how compelling someone to testify is at all productive. If someone isn't willing to voluntarily testify, then what makes you think their testimony will be reliable and true? If I were sitting on a jury, I would take the testimony of someone unwilling to testify with a large grain of salt.

As for acting on certain kinds of crimes, the job of prosecutors is to gather evidence, and find people willing to testify, to support their case. They already do this, sometimes offering things in exchange for testimony (such as immunity for their part in the crime), to get witnesses to cooperate and offer testimony that can be believed. If they can't get someone to voluntarily testify, however, it seems counterproductive to me to toss them in jail to punish them for not cooperating; from the outside, that to me makes it look like their case isn't very good to begin with.