It's an airborne virus. It doesn't only take recirculated air to spread it. The reason they issued the travel alert is because of the concern that it will be spread globally, not because of the nature of air recirculation in an aircraft.

The couple in Canada who died "casually transmitted" it to their family upon return from their trip to Asia. The infected family members in Toronto are now in isolation and quarantine.

The WHO had to issue the global travel alert because it is an airborne virus and could be spread throughout the world so easily. The WHO doesn't issue these alerts lightly. The WHO has issued such an alert three times in its history.

Plus, hospitals also have recirculated air which adds to the problem, but it's my understanding from reports, that this bug is a person to person airborne transmissible virus. Why else would they have so urgently sent mask and goggles to the affected Asian countries and set forth the protocol for health care workers to wear them?