This question has been asked many times over the years.
The reason behind the removal of a pacemaker is due to the fact that a cremator can reach a very high temperature of 1000 degrees Celsius. Pacemakers contain batteries which contain gas, depending on the year of manufacture these gases could be hydrogen, iodine or lithium, in some circumstances they could house zinc oxide, nickel cadmium and plutonium. These gases are explosive and despite the small size of batteries installed can cause major damage to the walls and door of the cremator chamber from flying metal.