easyJet also require APIS information, at least for flights to and from UK, I can't remember about internal Schengen flights as it's been a while since I booked one, preferring to do my European travel by rail where possible.
I think it's laziness on the part of the programmers. Whereas is it is not always required, programming the system to only ask for it where it is mandatory would require extra coding, so they put the onus on the pax by saying it must always be provided.