They pitch it as a “discount” if you complete it and meet goals.
A good case study of this is world of warcraft's rest system when it was first introduced in beta vanilla back in the day. Initially it was called a penalty for playing too long; you would begin to receive less XP. Players didn't like this, so they rereleased the system and called it a rested XP bonus. Players loved it.
It may be manipulative, but if it makes people happier and results in higher usage rates for the system, then why not?