Unfortunately it's been an issue on every cruise line which does not have a program in place to curb it from happening.
Carnival just implemented their system fleetwide a couple months ago. As posted by John Heald (Carnival's Senior Cruise Director), "Under the new system, shipboard team members monitor sun lounge usage and if they observe a seat that contains a towel or personal belongings but appears to be unoccupied, a notification is placed on the chair indicating the current time. If the chair remains unoccupied for 40 minutes, the contents are removed and held for the guest's safekeeping."
Without this in place it was not unusual for all chairs to be taken very early in the morning and be occupied by only a towel for hours on end. The new system isn't perfect but it has helped a lot.
Norwegian Cruise Line has also implemented a similar program over the last few months too.