Why Pacific airfields still required defensible garrisons

Did the Washington Naval Treaty restrictions also prevent Pacific airfields?

No, but building airfields without enough aircraft and troops to defend them could create liabilities rather than useful bases.

The Washington Naval Treaty did not prohibit Pacific airfields. An airfield without enough aircraft and ground defense, however, could become a liability rather than a useful base. In Malaya, defending scattered airfields consumed battalions that otherwise could have been concentrated for the main land campaign. If those positions fell, the defender also had to disable them quickly enough to prevent the attacker from using the runways.