'Controlled' is honestly pretty accurate. In principle anyone can fork WebKit; in practise it would take major investments on the three fronts of engineering effort, standards politics and mass user adoption to make a WebKit fork that "matters" - one that web designers would worry, and have to worry, about being compatible with. If you don't have both the resources and the desire to do that, then you have to play nice with Apple and Google - or with one of their two remaining competitors.