Snowden's actions were effectively an attack on the mechanisms of governance: if they let him off the hook, they fear they will encourage more people to act like him, and that affects the solidity of the lever of governmental power.

If government is a tool, it's a rare politician that votes to make the tool less reliable. Note that this is distinct from changing the scope of what the tool can affect: this is about the reliability of the tool. Even politicians that were in favour of what Snowden did, who thought he contributed positively, and voted against surveillance, would still likely baulk at making life too easy for more people like him. That's because government is supposed to flow from the top down, and policy change is not supposed to emanate from the machinery. The machinery is supposed to execute the orders of government.