Perhaps they're thinking you don't dive into training camp by handing the most important position on the team to a guy who was drafted in the 7th round and has never taken a snap in the NFL. Quibble with the signing of Harrington over other available free agent rental QBs, bemoan letting go of Schaub, but no GM or head coach in their right mind is going to turn the keys over to Shockley before camp even starts. He'll have plenty of time to supplant Harrington.
Every QB drafted into the NFL is All-something or other. I'm sure Shockley's good, but there's a reason he fell to a team that already had two starters, in the second to last round of the draft.