They're not "charging" you to use different services. They're taking their $8/day WiFi access charge, and reducing it by 75% if all you need it for is to send iMessages.
What you just said makes it seem like you think Southwest is charging people $2 on top of their $8 fee to use WiFi, which they are most certainly not doing.
Easy: Make sure "economy" internet includes enough to cover most people's needs, most of the time. Price and market "full-service" internet like a luxury so most users will select the "economy" plan.
Sell inclusion in the economy plan's whitelist for a private, negotiable fee (measured in millions). Sell exclusivity per category (not outright, but by offering reasonable fees to only the highest bidder). Big players don't have to worry about competition, so their R&D costs are lower and their stocks are safer (and more attractive) investments. ISPs win, large internet companies win, small-government and pro-business voters win, the market wins. Large companies which are in themselves platforms (Google, Amazon, Apple) become the only option to reach consumers, so they can take as large of a percentage as they want. Consumers lose.
This is precisely the nightmare scenario motivating internet neutrality protections.
What you just said makes it seem like you think Southwest is charging people $2 on top of their $8 fee to use WiFi, which they are most certainly not doing.