They're just in a different position... In the DOJ, encryption gets in the way of their job--catching criminals... while the penalties for a backdoor fall on others (those responsible for the data). There's no downside to them personally (and very little for the DOJ itself) for getting a backdoor.
The opposite is true for those not in the DOJ... the protection for our data is compromised.. and in exchange, the DOJ is able to prosecute some criminal we've never known, met, or had any contact with.
To those in the DOJ who deal with criminals everyday, their ability to prosecute criminals is of the upmost importance to them personally... and for the rest of us, unless we're a victim of a crime, there's only downsides to a backdoor.