These events are organized and promoted by developers themselves to promote their games and brands – and they hire the teams or players that will take part in competitions.
As a rule, this hiring process takes place through a partnership and disclosure contract for marketing purposes in which teams commit to taking part in competitions by hiring players and technical staff.
Since these are private competitions to market and advertise brands and games, participating teams are carefully chosen by developers as marketing partners, to ensure that only serious teams that indeed want to contribute to the sector are present at competitions.
In other words: these are not public competitions, but private ones, in which developers’ freedom to hire is ensured by law (Creative Commons art. 421 and 422) and in which they may select teams and hire them to take part in competitions.