I will answer for Candy here - her Dusty doesn't bother about it because he is focused on her, not the distractions flying about the park - if he does go after it then she calls him back and he responds - it simply isn't a problem.

The point Candy made either earlier in this thread or on another is one of the most important details of dog training - no matter what method you use to train - get your dog to the point where he/she can obey commands in the house, then move to the garden, then move to the front garden, then practice when you are on the street. Every time the dog reaches 100% reliabilty in response to a command in a certain situation (and you have to start off small - the dog must learn the correct response in a distraction free environment) then you move the situation - tiny steps, more distraction until you can expect a response in a mega-distraction area like the park.

The other point she made that I think is vital in dog training is the use of correction - if you know that your dog knows the command - and can respond 100% in a given situation ( say at home) then you have to be fair about the amount of distraction you introduce in the next step but you must be prepared to correct a response to a command that the dog knows that it chooses to ignore. It is a fine balance but correction has a place - firm but fair.