I had always understood 'there's no such thing as a free lunch' as a expression to demonstrate the economics concept of opportunity cost - whereby even if the lunch is fully paid for, one loses. Saying free or available rather than busy may be considered a more positive enquiry. It may also simply mean that you expect the person to be busy rather than free, rather than the other way.
Understanding the Context
The fact that it was well-established long before op's 1930s movies is attested by this sentence in the transactions of the annual meeting from the south carolina bar association, 1886 and to.
