This is common. Companies that provide the data for offline use tend to have a separate licensing and subscription fee structure. Companies that provide the API tend to forbid offline caching/storage of the data.
The service is whatever is described in the contract you agree to when you purchase it.
If you don't like the terms of the contract, you can always try to negotiate an alternate agreement. Or you can choose not to purchase the service.
The seller isn't obligated to provide their services on your terms, just as you're not obligated to purchase the seller's services on their terms if you don't agree to them.
The price that captures that value would have to be much higher in the model where you only need to access the database at some interval (let's say weekly), and that's not necessarily any more palatable.
It’s an interesting service that I would be very interested in using in providing a service of my own. And I’d be more than happy to pay for it, but those terms are a non-starter, at least for me.
The year is 2040. There’s no running water. Grocery stores mandate that all purchased liquids must be consumed prior to leaving the premises.