Because you have extensive knowledge in the area of how many people use parks, at what times and in what way? I'd say 300 people running the same route every week would count as significantly more intensive than the park's paths were designed for.
I know that running clubs - the things that existed before parkrun - often have to pay (or at least come to an arrangement with councils about parks and pavements) if their groups get too large. Again, I have no problem with that.
The sponsors seem to be paying for a fair number of employees and significant national branding. Perhaps for another 10k between them they could pay to keep local councils happy.