One group of people feel professional negligence resulting in death should be punishable by jail. Others feel that pilots need this noose over their necks removed so that the truth can be known when there is an accident. When pilots can be free to give testimony and perhaps may not do so if the threat of being jailed may depend on what they say.
Taiwan authorities were close to jailing the crew of the Singaporean 747 that crashed on takeoff in Taipei. But they were persuaded otherwise by the outcry from pilots worldwide and the Singaporean government. They probably would've been jailed had they been pilots on a Taiwanese carrier.
However, jailing pilots for say five years after a crash that is proven to be their fault. That doesn't decrease the risk of air transport. Very few pilots actually want to crash and he/she wouldn't be able to predict whether he/she would even survive such a crash.
A jail sentence serves no real purpose here in my opinion. It doesn't deter crashes from happening and it's of no help in obtaining reliable testimony from the flight deck.