Here are the flights tables for services from Denmark, Finland, Sweden, and Norway to Rhodes and Kos for this summer. The Thomas Cook and TUI flights are all charters, only one, two or three week bookings, and no one-way tickets. Why, I don’t know. There are no legal restrictions, the whole route is within the European Common Aviation Area. The two groups of companies have subsidiaries in other European countries that sell seats for longer periods or just one-way, so they already own the software to handle it.
Some holes in the service from Denmark after the collapse of Primera last year.

Flights from Scandanavia and Finland

Flights from Netherlands

Flights from Austria

Flights from Switzerland

Flights from France

Flights from Germany

Flights from the UK

Flights from Belgium and Luxembourg