No, it's not a legal requirement. But an outright ban is just unrealistic and isn't practical. It's like them deciding to only sell food outside the front gates (yes, I know food and cigarettes aren't the same, but smokers are going to need to smoke and the reality is when someone needs to smoke- and once you are a smoker it is a need, it's an addiction, right or wrong and even though it's their own doing- the reality is they are going to need a cigarette and a majority are not going to spend 10/20 minutes or more round trip to exit and re-enter the park to do so).
People can rant and rave and be on as a high a moral horse as they want over smokers. It's not going to stop people smoking. And banning it from the park will not stop people smoking in the park. Now without dedicated areas there will be random pockets of smoke wherever as people try to sneak one.