More details: I have reproduced this with maildrop 1.4 as well. Here are my compile flags:

I'm calling maildrop from postfix with:

maildrop -d ${user}@${nexthop} -f ${sender} ${recipient}

To reiterate, ktrace shows that only the first few bytes of the userdb are being read. After that, the file access pattern suggests that getpwent() is being called.