Why are prisoners not allowed to commit suicide while in prison?

Don't get me wrong. I know that there is a HUGE difference between a situation in which a SUSPECT either commits suicide or attempts to do so while in jail and a convicted offender who does the same thing while incarcerated. After all, the suspect hasn't been brought to trial, yet, and his/her guilt or innocence still needs to be determined. In situations involving convicted criminals, though, I can't help but think that they should have the option of committing suicide. After all, if they kill themselves, doesn't that leave more room in the prisons for future convicts and doesn't that also save the taxpayers money, since that's at least one less person they will have to support?