Slowdown makes sense if you are in the business of selling or owning it. There is only a certain amount of bandwith available to any ISP, and that bandwith is available for all. Sure, the argument can be "get more bandwidth", but there is a physical limit. If a few take it all, others have none. By limiting bandwith (speed) if too much is used, they are able to let someone else use some. In most cases, one will never see this slowdown, but may see their own computer slowing down because of memory leaks or background jobs that will be blamed on the provider.
Many will not believe this, however.