They summon the unfortunate heroes from around the realm, whether they were shopping, sleeping or taking a dump, onto the Summoner's Rift to fight as Pokemons.
The "champions" then fight hard with blood and sweat because they are told that the only way to get out is through destroying the enemy team's nexus.
Even though after the first group summoners is pleased with the entertainment and thus has since left and do something else, another pack logs onto the "League of Legends". And so the champs are still unable to leave and are forced to keep fighting for summoners' pleasure.
It all makes sense now.