You see a "Lead is Created" trigger for Smart Campaigns but no "Contact is Created."
This trigger that you see, "Lead is Created" is referring to the Marketo "Lead", and it does not incorporate the Salesforce Lead or Contact Objects. When a record syncs down to Marketo from Salesforce, Lead or Contact, it will be created in Marketo as a "Lead", and will activate the "Lead is Created" trigger. Salesforce Contacts and Leads will both fire the Marketo "Lead is Created" trigger when they are initially synced to Marketo.
Alternately, you may actually see the trigger called "Person is Created". This is the same as the "Lead is Created" trigger and will respond to both SFDC Leads and Contacts.