I'd guess a lot of the regular mercenaries aren't terribly different than "contractors" doing it for the money. The prisoners are doing it so they can out of prison.

Also I would guess the bulk of the prisoners were recruited before there was any knowledge of how bad it was going to be for them in Ukraine, and even more recently recruited prisoners don't have any thing like access to the information we have.

Not to mention, they're prisoners with probably a lifetime of bad decisions behind them that led them to prison. Just one more in a long line of unwise things they've done.