Why should troops make a population jubilant...

[Q]

I understand your point and you recall the happiness system in SE III correctly, but I found it just too easy to keep the population happy in SE III: reduce the production for a few turns and they changed from rioting to jubiliant!
I like the system in SE IV better: if you have only one colony in a a system that is riotimg and no troop transport or other ships around you will have a serious problem.
And think of the troops more as a police that protects the population, then you can explain why they react positively to their presence.