The fee is to see the recently renovated Portico de la Gloria (PDG), NOT to enter the Cathedral. The PDG recently opened after a 10-year renovation, costing millions of Euros. The fee is to recoup the costs of renovation and to assist paying for the renovation, after about 150 years, of the entire Cathedral.
That effort is ongoing and is intended to be fully completed before the next Holy Year in 2021. The external scaffolding on the west side, main portico and entry of the Cathedral is GONE.
By tradition and longstanding Church practice, entering the Cathedral for worship, or to view any central part of the Cathedral devoted to worship, is free. Only voluntary donations are accepted. All persons are welcome to visit and to worship.
Those parts of the Cathedral, accessible from outside the 'sanctuary' have access charges. This is because these places are more museums than they are places of worship, per se. They just happen to be attached to or directly adjacent to the ancient Cathedral.
These ancillary places for which entry charges apply include:
Crypt and archaeological digs under the Cathedral
Portico de la Gloria
I could be argued that the PDG is so highly valued as a cultural and architectural landmark that it could be in a museum. In that setting, you would pay to enter most any museum. However, moving it presents obvious problems. So it is a museum exhibit in situ.
Just to add that after so long the scaffolding is down and the cathedral looks gorgeous. They have added copper banners in some places on the statues high up, that shine when the light catches them. It's really stunning.