Slack is essential, but an excess of wasteful idleness is not. True leadership is being able to tell the difference.
Scrum Roles and Story Commitments
The Product Owner prioritizes the Product Backlog, but only the Development Team may estimate stories. The team uses these estimates, along with their estimated velocity, to determine how much work should be accepted into each Sprint.
This only seems like a problem if you take a narrow view. The project has milestones, release dates, and other targets that can be compared against current progress at periodic intervals. If the team is not on track to meet the necessary management targets, then this is good material for organizational dialogues and team retrospectives.
Slack Isn't Always "Waste"
Please note, however, that simply saying the team is under-committing is not the same thing as actually having a team that has large buckets of unused spare capacity. The goal with Scrum is sustainable development, which requires some amount of process slack. It's up to the team and the organization to reach an understanding of how much slack is necessary, and how much is "waste" (e.g. muri, muda, or mura) in the Lean sense of the word.
Remember, 100% utilization is not the goal. If the team is already working at it's optimum sustainable pace, you may need to examine your project's expectations, processes, resources, or scope to find other ways to address your system constraints.