I think you are jumping to conclusions here. There are plenty of respectful reasons why a candidate would want to look at the code. After all, the code is what he/she is going to work with. So doing this is as logical as getting to know your future colleagues, workplace and corporate benefits.
I see two main reasons that may prevent the company from sharing the code. First: their security is below standards: e.g. hard coded passwords to the production database in code. Second: their business is extremely sensitive to security(e.g. they are an anti-malware software company or a financial institution), so they must run a deep background check on candidates, which is time consuming, so they only do it right before offering the job and not earlier in the hiring process.
In any other case, an simple NDA would be enough.
