To add to mariomike's comment to clarify further...
Patients need to be stable enough to make it to the stroke center. If they are CTAS 1 they are not stable, therefore must go to the closest hospital.
If a paramedic decides they are stable enough to make it to the closest center, they would come in on a CTAS 2 most likely so that their patient falls under the protocol.
CTAS improved things. It standardized communication between paramedics, emergency medical dispatchers and the hospitals. CTAS 1 are transported to the nearest hospital
regardless of how busy the emergency department is, and less seriously ill patients are to be transported to the hospital providing the most appropriate treatment.
It wasn't always like that. There was an inquest, followed by a lawsuit:
"A tragedy in Toronto early this year ( 2000 ) became the flash point for a health care system in crisis.":
http://www.cjem-online.ca/v2/n3/p212"Later that day, I issued a directive to the land paramedics of the City of Toronto Emergency Medical Service (EMS) indicating that, under specified circumstances (see Table 1), they should transport patients to the nearest facility regardless of hospital bypass status."
Same call:
"The legal duty of physicians and hospitals to provide emergency care: ACCESSIBILITY OF HOSPITAL EMERGENCY SERVICES HAS BEEN an issue of increasing concern and was recently brought into public focus in Ontario by the tragic death of Joshua Fxxxxx, whose ambulance was redirected from the nearest hospital.":
http://www.cmaj.ca/content/166/4/465.full "The new system, implemented province wide in October 2001, has standardized communication between paramedics, dispatch staff and hospital emergency personnel by having them use the Canadian Triage and Acuity Scale
(CTAS) to evaluate and describe the needs of patients."