Re: Why
You aren't comparing like with like. Inmarsat uses satellites in geostationary orbit 22,000 miles away. There is only a dozen of these satellites spread around the earth so they are expensive what with high contention and limited bandwidth. Starlink satellites are in low earth orbit constantly moving over the sky. That means that regardless of where you are on earth (barring the poles), you're going to have a multitude of satellites overhead. You could be in the middle of the ocean, a desert, a city, anything with a clear line of sight and there will be one overhead.
Now it may be that the 1st generation of Starlink terminal (dish and box) isn't so great if you mount it to a boat because it is not gimballed & motorised. But if you're stationary in a marina, a campsite, or a cabin in the woods then there is absolutely no reason the existing dish shouldn't function exactly as it does at home - lock onto a passing satellite, handshake, authenticate, internet happens. Any additional charge imposed for "roaming" is nothing to do with any technical issue, it's merely a cashgrab.