It the same monopoly that a town center has in term of political activities. Anyone could easily choose to switch to hold demonstrations somewhere else, but the result of holding it where only 8% of people will see it will make that less of a choice for the ones organizing the event.
As a society where I live we recognize the benefit of political diversity, and we acknowledge that the disruption that such political activity bring is usually minor. As long it does not cause prolong and significant disruption to everything else then the benefit of giving everyone equal right to demonstrate in the town square is a benefit.
I do agree however that Browsers are more similar to regular monopolies worries. One company in control of the market will increase prices, decrease innovation and create perverse market incentives.