Actually, it has nothing to do with exploitation. You don't have to join Medium. And you don't have to pay them. It's got to do with the banking laws in different countries.

The banking laws in various countries determine whether an organisation like Stripe will do business there.

For instance, when Itried to transfer money via one of the cheap money transfer services to South Africa, I couldn't. That's because South Africa does not permit money to come into the country in any other way than through a conventional bank.

As a consequence, a company like Stripe deals only wiht countries that have a particular style of banking system.

Medium probably uses STripe because Stripe is chaper than paypal - it's a business decision, nothing to do with being colonial.