Why? Because the city council hates trees. Really, that's about the sum of it. The city council has been attempting to sell the park for several years now - yes, that's right, I live in a town that's trying to sell its own town square! - to companies that would like to turn the park into a parking lot. But the attempted sales were blocked because the city council hadn't completed the required environmental impact report. That report is now completed, so the city is once again trying to sell the park. One of the environmental impacts the city now fears could block the next attempted sale is the fact that it is illegal to destroy the habitat of certain rare birds by chopping down trees that those rare birds are nesting in.
To make sure the trees can legally be chopped down to build a parking lot, the city has wrapped the trees in black netting for the foreseeable future, so that those rare birds can't possibly nest in these trees. In other words, the city council has reacted to the environmental laws intended to prevent the destruction of rare birds' habitat by intentionally destroying the rare birds' habitat even sooner than this habitat would have been destroyed with no such environmental laws in place.
Due to the fact that the town is surrounded on two sides by rivers and on all four sides by levees, the town only has two roads that lead in or out of it in a total of four directions. Those two roads (Highway 20 and Highway 70) intersect in the middle of town, and this park is located at that intersection. In other words, anyone who drives through the town from any direction can't possibly fail to notice the park full of netted trees.