1. Cheaper to just execute them than to pay for their needs for life.
2. As long as their alive, their terrorist friends may try to get them released by taking hostages & trying to bargain for their release. (Happens all the time, IRL)
All convicts should be "incentivized" to work in order to pay the cost of housing them and pay restitution to their victims - then it doesn't make any rational sense to kill them.
Execute convicted terrorists & you take away one incentive for hostage takers.