I've always thought of it as the turn-style in front of the theme-park. Without that turn-style, the theme-part is economically worthless no matter how good it is.

Yes, that's the idea, but keep in mind that "economically worthless" has a literal meaning that is almost opposite of what you are trying to say.

And not only that... but a theme park without an entry fee is still a money-maker via snacks, drinks, souvenir cups, and extra-cost rides and line-skipping.

In much the same way that a company can produce an open-source product and charge for support, feature development, consulting, data feeds and sometimes SAAS operation.

I just mean you can't charge for it directly. "Utility" is perhaps what you mean. Note that the most important achievements (scientific and artistic) in human history were economically worthless for their authors, but have had enormous indirect positive economic benefits.