It would be a bit counter productive to steal from the customers of the ATM's. If it ever comes out into the public, that the private key was compromised on the ATM side, the company would be ruined. And believe me, the effort to get these ATM's in the field, and running, will be enough motivation to keep them honest.

BUT you get dishonest employees and it could be hacked, so it's better to get the coins to a safe cold storage, too eliminate the human factor in this scenario.