Many of them were under recruited for valid reasons (injury concerns, size, work ethic, bad program, etc.) Why offer a scholarship when they are willing to come for free? They get a full year to evaluate them. If they are better than expected and improve, you offer up a scholarship. If they don't have what it takes, the coaches can cut them.
These are the players that have earned scholarships that were originally walk ons:
QB Joe Critchlow
WR Dax Milne
WR Talmage Gunther
DT Bracken El-Bakri
LB Payton Wilgar
DB Austin Kafentzis
K Jake Oldroyd