Review work is not always done by senior researcher (e.g., professors). Senior researchers often hand this down to PhDs. Having 3 to 4 reviews by nice junior reviewers doesn't sound very critical.
So PhD students writing the initial review is not weird - it is an expected part of their training. As is the supervisor going over the review and providing constructive feedback. As is the review being submitted under the supervisor's responsibility, with credits (mention in proceedings) to the student for acting as a subreviewer.
Yes, there are ways to abuse this system and yes, abuses do occur. Any system for gaining job prestige or workload reduction is a target for gaming. This doesn't mean the system should be thrashed, but it does warrant additions to curb excesses.
The sibling comments about making sure that most reviews are written by senior researchers also make good points. That should be checked by the program committee or editor.