No system is secure without physical control over the equipment, in the form of watchful eyes and chain-of-custody throughout all the relevant processes.
It's not the lock on the machine that matters, it's the person(s) charged with keeping eyes on the machine.
Given that the machine will be alone in the voting booth with the voters, there's no way for people to keep an eye on it. Hence the need for locks.
One might argue that allowing people private access not only to their own ballot, but the device that stores all the others, is a bad idea. I'd be inclined to agree. In optical-scan systems or even manual paper-ballot systems, voters mark their ballots within a private booth but then insert them into the ballot box in view of the poll watchers.
Still, having a machine which is well secured with locks that can only be opened by trusted people, and which is within earshot of poll watchers, is probably adequate. Someone with the proper low-security key may be able to tamper with a voting machine silently. Someone without a key isn't going to be able to force a well-designed locking system without being heard.