I think FR get away with the online check-in fee because sometimes it's waived in a sale campaign. So it's not always compulsory.
The online check-in fee never applies to
advertised fixed-price (not % discount) sale fares. FR only ever advertise promotional fares (i.e. those in the current promo on the homepage). They never advertise non-promo fares, which means they can slap an additional £5 fee on them without violating various rules about only advertising the achievable fare.