From the easyJet website:
API is currently required on the following routes:
- all international flights to/from the UK (including routes to/from Germany from 8th April 2013)
- flights to Spain from UK and Morocco
- flights to Italy from Morocco and Egypt
- flights to Switzerland from Kosovo, Turkey and Morocco
- flights to Germany from Morocco
- flights to Portugal from the UK
- flights to Russia
And indeed, on the routes between the Schengen countries API is NOT required. But please, do not accuse Ryanair of malice. As the process of omitting API data would require some changes to the website booking system, it would involve some costs. Paying the software provider for something like that? No way!
. Could be as simple as that.