Work holdover/mandate

At the hospital I work in, we are mandated/held over fairly regularly.  If I am held over I end doing a 16-hour shift.  The last time I was held over I did not clock out until 2 am because I was waiting for my replacement.(she was a float nurse and was finishing on her floor).  I contacted our house supervisor and she allowed me to come in at 11 am the next day.  This happened about a year ago.  I have been mandated multiple times since but fortunately have not had to be back the next day.  We received an email yesterday stating that if we are held over we are still expected to come in at the normal time the next day.  We can make arrangements with the house supervisor to come in later but it will not be considered a holdover at that point but will be considered a shift swap.  Which basically means they would be able to hold us over more frequently.  I feel like this is so unfair and borderline illegal.  Any thoughts?  I feel like we should at least have 8 hours in between and if it's only 8 hours it should still count as a mandate.