ADSL and DOCSIS (the system VM use) are shared services whereas a leased line runs directly from a business premises (usually) to a carrier point of presence which will have larger capacity or even multiple backhaul links from it that your average street cabinet will not have but the exchange might.
However companies are happy to pay far more for their connectivity than the average consumer.
Traffic management is therefore necessary to prevent those who want to max out their connection from causing other people's connections to slow though it does not always work out that way.
In the same way if you work within a company using a corporate LAN and tried to do what you dd on your home connection I can pretty much guarantee that sooner or later a member of your IT staff or your boss would be by your desk asking you to desist because your activity was slowing the network down.
Virgin (ADSL) => Namesco => Newnet => O2 => Plusnet => Zen => Newnet => Zen => Freeola => Vivaciti (using O2 Wholesale DSL) => Xilo (C&W Wholesale) => Xilo (O2 Wholesale)
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.