It would seem like the prosecution must have had a strong case, or they would have fought it.
The perjury is a felony in and of itself and the death that resulted from commission of that felony would be a capital murder in a lot of jurisdictions.
The witness tampering would surely inflame jurors and increase the likelihood of a guilty verdict.
Although it surprises me (but does not disappoint me) that they were charged, what they have now is a bargain compared to what they would have received in a trial.