It's possible that Megatron lied to Shockwave: perhaps to test his loyalty?

 

I actually think Megatron was lying to all his minions in order to keep his deal with Sentinel secret. He also orders to the Dreads to attack Sentinel, even though if they'd caught him or killed him before getting back to base the plan would be ruined.

He was probably counting on the Dreads dying, as well as Optimus getting the part despite Shockwave's best efforts.

 

If you want someone to get something, DON'T SEND THE LEGEND WITH THE GIANT HELLWORM AT HIS BECK AND CALL TO PROTECT IT.

 

Consider; Megatron asked Shockwave to go to Chernobyl and retrieve an item that looked like a pillar. However, the only thing there was the Ark engine piece. Shockwave may have decided that, since there was no pillar thing, he may as well pick up the next thing he saw, and thus was not as inclined to defend it when Optimus snatched it.