The OP / Title is a lie. As there is no law forcing people to release their tax returns ANYONE can 'get away with' not making their tax returns available for public viewing.
Actually, you're right, there isn't any law that says a candidate must release their tax returns. However, there IS a precedent that was set in the early 70's, and has been followed since. Every last candidate since the early 70's has released their tax returns with the exception of Trump.
And......................being audited is no excuse. Nixon released his while they were being audited.