I get calling off a chase when the people being pursued are in a car and they're driving so dangerously that they might knock over loads of people at once, but wearing a helmet or not, a bike rider is as big a danger to the public, and if they choose to remove their helmet that's their decision and theirs alone.
It's a policy that protects the officers as well as the suspects. No police officer wants to cause someone to be killed as a result of a pursuit, nor do they want to go through the inevitable hell of an investigation and potential trial after doing so.