Donating money to help build a park does not give an individual or a company a say in how that park will be used. If Miller donates $100,000 to the city for the park*, they will not be able to come back later and say that they do not want a particular group to have the right to meet there. The parallel might be if you were to donate money to the Occupy Movement or to the NRA you would not then be in a position to dictate to them on the issues they could support or protest. You are making a donation - either for a general campaign or for a specific action item. In the same way, someone may choose to give $50 to help build the park, or they may give $50 to help pay for a specific item in the park.
* There is a difference here in making a donation versus selling the naming rights, such is often done for sports stadiums. In that case it is not a donation, but rather a financial transaction in which the "buyer" may receive advertising and potentially an exclusive right to sell their products at the stadium. That approach has not been suggested for this plaza.