There is an expectation that the parent will return.
No. That they are required to take them back doesn't lead to the conclusion that they expect the return. Most maternity-fill I see is by temp-to-hire. They expect to fill the position permanently, but have to be able to clear it out, if the employee returns.
Your premise is that employers are dumb. They know the employees don't return. They plan on the employees returning. That's dumb.