Please note that Parental Control offers basic app and website blocking for free without a time limit. The 14 day free trial refers to Parental Control Pro, which adds extra features.
Parental Control Pro requires continuous maintenance beyond updates, upgrades and support. The website lists behind the website categories have to be maintained and updated with user suggestions.
A one-time purchase would only be sustainable if updates and support were limited to 1-2 years and no new features included. We would have to hold improvements back for paid upgrades and you would end up with an outdated, unmaintained app and no support eventually.
The subscription model makes the development process more efficient because we can invest all our effort into a single, up-to-date version. This allows us to respond to user suggestions in a timely manner and it enables continuous updates, upgrades and support.