Exactly.
"We're not throttling the startup, Twitter has paid for priority access!"
Aside from the pure commercial, profit-driven shenanigans this will produce is the door it opens. If you can "slow" traffic down (by whatever cleverly named mechanism), can you slow it down to the point that browsers would time out before receiving any useful amount of data?
Could a website be blockaded from public view without any kind of court order to shut it down/suspend web access?
Creating an analogy for that has to go absurd places.
See, here at the print shop, we produce and bundle the newspaper. Advertisers, of course, take up a lot of the total page space in many newspapers. Well, we've streamlined our operation and now have Priority Printing Services™. Our advertising customers can pay for this and their ads will be printed by the machines, guaranteed to be seen by every reader. We will continue our Standard Tier*, naturally, for all of our customers!
*Standard Tier services feature enthusiastic interns armed with a set of colored sharpies who will replicate your advertisement on as many papers as they possibly can, but no assurances can be made they will be complete or accessible on all papers circulated.