It sounds to me as if you aproached the subject wrong. The legality of airsoft relies alot on how you present it and what you call it. Lets use an example of paintball that is well known:
Paintball Gun
Paintball Marker
Which do you think the police are gonna be lighter on? Also, when you call, don't just ask them a spot to do it since, it is illegal to fire it off within city limits unless on your property etc. etc. Call and say something like "Hi, I, and a group of my friends have a sport like paintball we'd like to play that has a low velocity, doesn't break the skin, and we will use all safety procautions etc and play outside of city limits, is this okay (or may I have permission).
EDIT: As you dad stated, the officers on patrol may not know about it, but police officers are to report in, where they are, what they are doing, the reason they're going to a call etc. Chances are, someone will be notifyed and will tell the officer what the people are doing and maybe just to check it out just to be safe.