It's not a case of limiting the novice shaman's power at all. It's how the rules are presented in the book.
Let's say your somewhat heroic shaman has slightly higher than average POW and CHA, both of which mark them as shaman material to the tribe. Let's say the character starts with 14 POW and 14 CHA.
During chargen they open Spirit Walking, and Spirit Binding is opened at 28. The critical range for a score of 28 is 3, and so the shaman can capture a spirit with a maximum POW of 9.
Let's say he spends some Improvement Points, and builds up his POW and Spirit Binding, to POW 15 and +17% respectively. So now he has Spirit Binding 46. By this time the critical range is 5, so as per the book he can capture a spirit with a maximum POW of 15. This would, incidentally, be the maximum POW of his fetch; any higher, and the fetch would either flee or control him.
Either way, Ash Ketchum would be so screwed.