I say this not to limit imagination but simply to offer a perspective. As noted CT doesn't mention Highports, or Downports for that matter. It only says Starport. Note the singular. As in the (main)World of a system has only one place the characters can land their ship. Yes, land. The implication is strongly suggested that the Starport is always ON the planet.
There's a reason or two for this I think. To encourage getting off the ship and wandering around the new world, meeting new life forms, and shooting... err, interacting with them.
There are no Highports and they aren't really needed in CT as all the player ships are streamlined. Or maybe there are Highports (some reference somewhere to cargo accepted in orbit is delivered in orbit, but cargo accepted on the ground is not delivered until on the ground) but they aren't (usually) meant for player character use. And anyway, if you accept cargo at a Highport in system Alpha bound for system Beta which doesn't have a Highport that surely doesn't mean you have to wait for one to be built so the delivery rule above doesn't really make sense.
In any case, I can see a simple rule like A & B have Highports, C & E don't being fine. Think I've long used just such myself, at least as a general guideline. It's just a game, and the Imperium (who runs the Starports) may have just such a definition (we already have similar service level definitions of Starports). And it fits with the shipyard split. So a world with a Class C Starport is a Downport and not only needs to upgrade the services before being labled a Class B Starport, it also has to provide those services at a Highport. So some systems may well have both, though I'd say only the best is properly THE Starport (with the other relegated to backup status, planetary spaceport, and not Imperial).
Likewise though I see other factors making more sense, but they can't be hard ruled and have to be left up to the referee to decide. Things like (some mentioned already) high-gravity worlds, high-law worlds, balkanized worlds, and harsh atmosphere worlds probably having only a Highport (of whatever service level) and no Downport for routine traffic. Or high-law worlds might just demand a Downport as being (perceived at least) easier to control.
Making the players leave their ship at the Highport and use local shuttles to visit the world is a good way to broaden adventure possibilities too. Otherwise they may just use the ship weapons or leave when the whim strikes them. Having to wait for local transport, clear customs, etc. can add to the game. And how else are you going to get them to use that Beanstalk? Play the tourist trap lure card?
