This is not true. Even if flying in a park doesn't violate some posted list, it could be a nuisance, and if your plane wanders beyond the park limits it would be trespassing. Flying near other people using the park would be dangerous, and could violate any number of laws, even if flying isn't listed as a no-no on the park sign.

As I said, not even close. There are some parks in which flying nitro-powered planes would be OK. Most parks (around here, anyway) are too small and, at least on weekends, too busy. Not one of those much-too-small parks has a "no model planes" sign, but that certainly doesn't mean people are free to do it. They don't have signs saying not to set off grenades or play touch football using live mortar rounds either, but it wouldn't be prudent to try it. The idea that you can engage in any activity in a park, or any place else, if it's not expressly prohibited is ridiculous.