I would say it's more like saying that driving on the highway is free because you pay taxes. I doubt anyone is buying a developer account specifically for weather API calls.
It's free like riding the monorail at Disney World is free; included in the cost of your entry ticket, and utterly inaccessible to anyone who has not paid.
We're not talking about "weather forecasts", we're talking about a specific set of weather forecasts accessed in a particular way.
Highways are mostly funded through gas taxes, and registration fees for EVs. Even foreign tourists buy gasoline, or drive an EV that paid its registration fee.
Also, the Disney World monorail is outside the ticket gates. You can ride it without a ticket.
Source: I live 30 minutes away from Disney and partially moved to Florida when I started working remotely to save money on taxes.