I found the offending page, a menu of mobile internet packets available as add-ons to mobile accounts with MEO, the largest landline operator in Portugal.
The same menu appears in the list of unlimited mobile plans.
So customers can choose from S, M, L and XL plans which offer unlimited voice calls to MEO numbers and a variety of data amounts:
S €14 for 1 GB
M €22 for 1.5 GB
L €32 for 3.5 GB
XL €60 for 30.5 GB
That's a wide range of rates, from €14/GB for low-volume users to €2/GB for power users.
The add-on options provide an additional 10 GB for a few selected services in that packet for an extra €5, the same discounted rate that the power users get.
As far as I can tell there are no restrictions on how you use the data in the base plan. A cost-sensitive customer who wants to use Facebook a lot but can't afford the power-user option can get the small plan and add 10 GB for Facebook at the power-user rate. If the user also wants to use WhatsApp a lot, they can add another 10 GB for WhatsApp at the power-user rate. If they also want to use YouTube a lot, they can add another 10 GB for YouTube at the power-user rate.
And they can mix and match these options however they like. If they want all the options, they can add all five add-ons for €25.
Everyone likes choice, right?