Don't think PC comes into it, although the reasons given to the pax sound a bit daft. From Easyjet's carrier regulations on their website:
If an adult is travelling with more than one infant under the age of two years, but no less than six months, one infant may sit on the accompanying adult's lap and the other infant(s) must occupy separate seats and be seated in a car seat in accordance with the above table.
and
easyJet does not operate an indemnity policy and therefore under no circumstance, under the Child Protection Act, must another passenger be asked to accept responsibility during flight of an unaccompanied minor.
Seems pretty clear for once, especially as the EZY site also defines the acceptable dimensions for a child seat to be used on board.