The developer overlooked some features of Steam's rules.
Recently, developer Tecopark accidentally made his project PICO PARK:Classic Edition permanently free on Steam. This was not part of his original plans.
The creator missed a feature of Valve's rules:
"I was planning to switch to a paid distribution model after the online support update, but I forgot that after switching from paid to free, you can't go back to paid! :) "
PICO PARK:Classic Edition is a puzzle platformer. The update added online play support, updated visuals, and brought various fixes.
Tecopark invited players interested in the first part to check out other games in the franchise (they are paid).