AMA's position is that you must fly safely and in accordance with the safety code, which does not prohibit flying in parks.

Local ordinances vary from locale to locale, and sometimes widely. You might avail yourself of a copy of the salient ordinances from your parks and recreation department (whatever they call themselves) just to be sure you're in the clear. Even if there is no extant prohibition against flying in the park, it only takes one complaint and an ordinance will be forthcoming.

Methinks that's what you've run into, a 'soccer Dad' who thinks the park is for the exclusive use of ball-and-stick-game types.