Claiming that the rest of the parks are usable as normal is ridiculous, taking a walk around the periphery of a music festival is not an enjoyable experience. One of the benefits of time spent in green spaces is the reduction in noise level which is out the window when this is going on. The noise pollution also has a major effect on the wildlife in the park which is usually another draw to parks for me.
Fundamentally, a park is not a good choice for a music venue. An empty field may be, but a park is not simply an empty field. It serves a purpose in the community which it can no longer do when it's used for this sort of thing.