I discovered this issue concerning command blocks when in survival mode:

What I expected to happen was:
Command blocks take time to be destroyed when punching them in survival mode.

What happened was:
They break as fast as leaves when using shears.

Steps to reproduce:

• Type /give YOURPLAYERNAME 137 64
• Place as many command blocks as you like
• Make sure you are in survival mode ( /gamemode 0 ) and break the blocks with your fists

I am not sure, this might be intentional. But it feels strange that only op'd players in creative can program command blocks, but players in survival can break them easily. They should have the resistance of obsidian, or at least of stone, to prevent players breaking complex redstone structures.